Answer:
(a) Eva endorsed the check, without making any further notations on it, deposited it and sued Maria for the remaining $15,000 she claims is due. Judgment for whom?
Maria's debt is settled (cancelled) because Eva accepted the check and did not write "under protest" or "without prejudice" when she endorsed it. This debt was an unliquidated debt since maria and Eva did not agree on the total amount due, but since Eva cashed the check without writing down any type of notation with respect to their disagreement, the debt will be considered settles.
(b) Instead of the facts in (a) assume that Eva wrote "under protest" on the check when she endorsed it and, after depositing it, sues Maria for $15,000. What result?
If Eva wrote under protest, then she is not accepting Maria's payment. She is entitled to cash the check because she might need the money, but she is making it clear that she doesn't agree with the settlement. Eva could win the suit (that also depends on other factors not specified in the question).
(c) Instead of the facts in (a) and (b), assume that Eva and Maria had a telephone conversation in which Eva agreed to take $60,000 in full satisfaction of Maria’s obligation under the contract. Maria then sent the check, with a letter referencing the telephone conversation. Eva wrote "under protest" on the check, endorsed and deposited it, and then sued Maria for $15,000. What result? Explain.
Unless Maria can prove that Eva agreed with the $60,000 payment, then a court would not even consider the telephone call and the outcome would be similar to question (b). The problem with oral agreements is that one party will say this and the other party will say something else. You need some type of proof in order for an oral agreement concerning a $75,000 (or even $60,000) dispute to be valid.