Suppose Uncle Sam charges your client with a crime. You're a cautious sort of lawyer, dedicated to exploring every option available to your client. It is early in the case, and so when the feds express an interest in talking to your client, you decide "Why not?"
After all, should there be enough evidence to convict, you'll want to explore all the avenues for relief from a Guidelines sentence that you can find. Should your client offer the Government substantial assistance, a court just might, if the Government asks, depart from the Guidelines.
Out comes a proffer agreement prepared by the United States Attorney's office. You tell your client his statements can't be used against him unless he testifies at variance from the proffer. But read the agreement carefully. Odds are you are limiting the defense you can assert on your client's behalf.
A Second Circuit case is illustrative. A defendant in a cocaine base case, a.k.a. "crack" in most circuits, signed a proffer. He then told the feds he was a crack dealer. After the proffer, the defendant decided to go to trial. His lawyer said in opening statement that as to at least one sale the Government had charged the wrong man. It was a case of mistaken identity.
The Government then introduced the statements made by the defendant during the proffer statement. The Second Circuit upheld the trial court's ruling admitting the evidence. U.S. v. Barrow, 400 F.3d 109 (2005).
Why? The proffer agreement contained the following clause: the statements could be used as substantive evidence to "rebut any evidence offered or elicited, or factual assertions made, by or on behalf of [the defendant] at any stage of a criminal proceeding." In other words, the defendant's lawyer opened the door to the use of his client's statement in his opening statement. The Court also held that the door could be opened by cross-examination of the Government's witnesses. The key factor for the court to consider is whether the defense has directly or impliedly made a factual assertion that could be rebutted by the statements made under cloak of a proffer.
I recently completed a trial in which the defendant had been proffered by predecessor counsel. I challenged a Government witness and the Government gave notice of its intent to offer statements under a proffer. Much to my surprise, the proffer contained the language recited above. In the end, I persuaded the Government that my cross had not opened the door to use of the proffer. It was a close call.
The court appears to view proffers as a contract binding parties to the clear terms of the agreement. When sitting down with the Government early in a criminal case, be careful not to give up defense themes and weapons as you explore a plea. It could come back to haunt you at trial.