Can Customs make me go back to return my electronic equipment or is it a scam?

No, the Customs cannot make you go back, and return the equipment.

What the Customs can do is one of the following:

• Let you in with the item, optionally after duty is paid, if required. If you don't have enough money, Customs typically will hold the item until duty is paid, but they will not detain you.
• Hold the item temporarily (for example, to analyze the content for drugs) - if clear, the item will be returned to you later;
• Confiscate the item (and optionally detain the traveler) if the item is prohibited and was not declared;

What you describe honestly sounds more like a scam than a typical US customs operation. If the "Diplomat" follows up with you soon asking to send $2000 to "US Customs" through Western Union, please go to the police.

To recap, with thanks to many commenters, there are a number of suspicious factors in this question:

• Nigeria - a country well known for scams of this type. Not that there aren't many legitimate people and businesses in Nigeria, but it's a factor.
• Customs does not detain people for days because of unpaid duty. Someone would only be held for days if they were accused of breaking the law. If he is accused of breaking the law, there are legal processes that are followed, formal paperwork produced, criminal charges filed, etc...
• Reagan National Airport only has Immigration and Customs facilities for a handful of flights on private jets. If he was on a normal commercial flight into the US, he would have had to go to a different airport to enter the US and clear Customs, and so would not be at DCA at all.
• People are generally not detained in airports for days. They usually are either sent home or transferred somewhere else.
• A "diplomatic person" middleman who you do not know seems to be doing most of the talking. Have you actually spoken to your fiance?
• Diplomats don't collect customs duty or detain people at US airports. That job is done by Customs and Border Protection.

There is a familiar scam called the Family Emergency Scam:

Scammers may pose as relatives or friends, calling or sending messages to urge you to wire money immediately. They’ll say they need cash to help with an emergency — like getting out of jail, paying a hospital bill, or needing to leave a foreign country. Their goal is to trick you into sending money before you realize it’s a scam.

We don't know enough to know exactly what is going on, but you should be careful. I would take steps to independently verify all information. Don't trust any phone numbers or information you are given by the people you are talking to. If they say he is being detained by US Customs and Border Protection at DCA, then look up the phone number for that office and call them yourself.

In the unlikely event that he has actually been charged with a crime, the authorities holding him should be able to confirm this fact; then you should hire a qualified criminal defense lawyer (or request a public defender) to advise him. If he has not been charged with a crime, then you should contact the police and not give any money or personal information to these people.

Out of curiosity, what are they asking you to do exactly? Presumably whoever is calling wants you to pay the $2500 immediately. What kind of payment instructions did they specify?

I don't want to worry or scare you, but— no, actually I do. I need to. You must be open-minded and clear about this, because the situation is incredibly suspicious. Let's sum up what we know so far:

• Your fiancé went to Nigeria
• Your fiancé came back "early" to "surprise" you
• Your fiancé ended up at a non-existent Customs office, and claims to be detained pending payment of duty on some "hardware for his work" from Nigeria (despite the fact that a person would not be detained in this scenario)
• You are being contacted by some "Diplomat"
• The "Diplomat" describes a scenario in which your fiancé will not be released unless thousands of dollars are paid
• No documentary evidence will be provided until the money is paid

Are you kidding with this? This is so obviously a scam. The only question is whether your fiancé is in on it. I have read in the news, on several occasions, about long-cons perpetrated by Nigerian scammers who went as far as marrying American women to gain their trust, then extract money from them in pretty much the exact same way.

Without knowing the details of your relationship (how did you meet? how long ago? and have you spoken to him over the phone as well as this "Diplomat"?) I am not going to sit here and declare that your fiancé has been lying to you the whole time, but I strongly recommend you think about it carefully.

In practice, of course, your first step is to contact the airport itself to confirm that the story is nonsense. Do not pay anything. Then contact the police, tell them the full story, and take things from there.

Besides the obvious red flags and inconsistencies in this particular story, here is another answer to address the generic case:

If a fee is required to bring back work equipment, this should be something that the employer of the traveler should deal with. Not the fiancee of the traveler.

If it is convenient, you can of course be the one to alert the employer that this problem needs to be solved.

I can't stress enough that this smells like scam.

The closes scenario that I could see happening are the following. Please note that your situation doesn't meet these, but I wanted to give you an idea of what may be "normal".

When someone comes in from an outside country carrying an item of large value, you should declare it using a form like this one: Customs Declaration. So long as items are declared then there is almost no way that CBP will detain a person. They will detain an item if the declared value is WAY off, or if a duty is due. They may also detain an item if that item is illegal. If items are declared then there is almost no way they would detain the actual person, even if the item is illegal. If the item is illegal they would either confiscate the item, or hold it till you leave the country and pay a fee. Depends on the item. Some items are illegal because of treaties and laws and no big deal is made over these. Others (like drugs) are a large deal, but again, if declared, the item is lost not the person.

When you don't declare an item, and it's found a few things can happen. If its normal stuff that just happens to be a high value, a duty is charged, and the stuff it held till the duty is paid. The person is allowed to go their own way.

If the stuff contains a controlled substance or item (doesn't mean Drugs, some things like fruit can qualify) then the item is confiscated, and generally a fee is charged. The person is not detained, but could face fines etc.

If the stuff contains items and it looks like your trying to smuggle in drugs, or other things. CBP will confiscate the items, and CBP will release the person. Even if you smuggled 900 lbs. of heroin in, CBP will not detain you. They will take the drugs for sure. Now what will happen is that the person will be release by CBP to other agencies like the FBI, or Immigration, etc. depending on what was brought in. AT this point however court dates are set, legal actions have been taken and things can get odd for the person because US law is followed, but generally speaking, so are the foreign laws. It gets weird, but you would be needing to contact an attorney. The state department (for US diplomats) will never enter into the equation. Foreign diplomats may. That's up to them, but usually not. Usually paper work, and what not takes over and either jail time or deportation happens.

At no time would the person or the item be allowed freely into the US if it looks like they were smuggling. CBP would detain the items and another agency would detain the person. US citizens would follow US laws around being charged with a crime, which means 24 hours to "initial appearance" (bail hearing).

So with all that in mind. I could see....

• Guy gets "pulled aside" and told that his work equipment had a problem getting though customs, and he needs to pay a duty. Then he is given paperwork stating the total amount he needs to pay including fees, and where he needs to pay it. He is then released, and has to go to some middle of nowhere warehouse to pay his fee and duty and pickup his equipment. For the days and weeks it takes to get all the paper work straightened out Guy can freely move about the country and get what ever legal advise he needs.

• Guy gets "pulled aside" and told that his work equipment contains something illegal. (Sometimes this can be very non-intuitive, specially with electronics) He is giving paperwork telling him what he needs to do to recover his items. He is then released. Guy can move around the country freely getting what ever help he needs to get his items back. Even if it means sending back to the originating country.

• Guy gets pulled aside and told that his equipment is being used to smuggle drugs into the country. Guy is given paperwork telling him where the items are being detained (maybe not specifically, but a receipt is given). He is then released to the FBI. Because Guy is a US citizen, he must be charged and appear before a judge within 24 hours. Before that hearing, many things may be restricted. After the hearing the judge would have ruled, usually, on the validity of the case, and rater there is enough evidence to continue, and also made a decision about bail. From this point on, normal judicial rules apply. He's a prisoner, either on bail or in custody.

Remember above all, that if Guy is a US citizen (it get's a bit weird when people are not as different rules get followed in regards to their home country), he can not be detained, or denied access without due process. That process may suck, but without a hearing he can can not be detained for long.

Also, because he is a US citizen, on US soil, the state department won't touch it with a 10 foot pole. If anything happens it's a domestic issue, not a foreign one.