Wis. Stat. § 904.08
Evidence of furnishing or offering or promising to furnish, or accepting or offering or promising to accept, a valuable consideration in compromising or attempting to compromise a claim which was disputed as to either validity or amount, is not admissible to prove liability for or invalidity of the claim or its amount. Evidence of conduct or statements made in compromise negotiations is likewise not admissible. This section does not require exclusion when the evidence is offered for another purpose, such as proving bias or prejudice of a witness, negativing a contention of undue delay, proving accord and satisfaction, novation or release, or proving an effort to compromise or obstruct a criminal investigation or prosecution.
Wis. Stat. § 904.08
While this section does not exclude evidence of compromise settlements to prove bias or prejudice of witnesses, it does exclude evidence of details such as the amount of the settlement. Johnson v. Heintz, 73 Wis. 2d 286, 243 N.W.2d 815 (1976). The plaintiff's letter suggesting a compromise between codefendants was not admissible to prove the liability of a defendant. Production Credit Ass'n v. Rosner, 78 Wis. 2d 543, 255 N.W.2d 79 (1977). When a letter from a bank to the defendant was an unconditional demand for possession of collateral and payment under a lease and was prepared without prior negotiations, compromise, or agreement, the letter was not barred by this section. Heritage Bank v. Packerland Packing Co., 82 Wis. 2d 225, 262 N.W.2d 109 (1978).