In federal court cases negotiations are not admissable to prove or disprove factual questions arising in the case. This is true in civil as well as criminal cases though the Federal Rules of Evidence appear to only take the civil cases into account.
It should be noted however that in criminal cases where there has been a proffer of evidence incident to an unsuccessful change of plea, portions of the proffer may be admissable to prover or disprove certain relevant facts pertaining to guilt if there is a jury trial.
Further, the facts agreed to in a plea agreement as well as testimony given during the Federal Change of Plea hearing may be deemed relevant at a trial and therefore admissable should the Defendant decide that it's in his best interest to disreguard a previous plea agreement and go to jury trial anyway.
See Below the Federal Rule on Compromise offers and negotiations.
RULE 408. COMPROMISE OFFERS AND NEGOTIATIONS
(a) Prohibited Uses. Evidence of the following is not admissible — on behalf of any party — either to prove or disprove the validity or amount of a disputed claim or to impeach by a prior inconsistent statement or a contradiction:
(1) furnishing, promising, or offering — or accepting, promising to accept, or offering to accept — a valuable consideration in compromising or attempting to compromise the claim; and
(2) conduct or a statement made during compromise negotiations about the claim — except when offered in a criminal case and when the negotiations related to a claim by a public office in the exercise of its regulatory, investigative, or enforcement authority.
(b) Exceptions. The court may admit this evidence for another purpose, such as proving a witness’s bias or prejudice, negating a contention of undue delay, or proving an effort to obstruct a criminal investigation or prosecution.