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.