Responsa על בבא בתרא 61:12

Q. A and B hired a tutor for their sons [for several terms] and paid him for one term in advance. Five weeks after the term began, A's son died. Since the tutor had to continue to teach B's son, he demanded that A pay him for the remaining term (or terms). A, however, claimed that when the tutor came to console him, after his son's death, he told him that he forwent the balance of his fee and that he would even repay him whatever he had received in excess of the fee for five weeks. A, therefore, demanded that amount from the tutor. At first the tutor admitted that he had forgone the balance of his fee, but claimed that his act was not valid since it was merely a verbal statement without a formal act of transference. Later, however, he completely denied ever having relinquished his claim.
A. If the tutor definitely admitted A's claim, and later retracted his admission, A is free from obligation to the tutor. However, he cannot collect anything he has already paid, since the tutor's promise of a refund was not accompanied by a formal act of transference. But, if the tutor's denial followed immediately upon his admission, or if the denial merely explained that the admission was really no admission, or if the admission was only implied but not definitely stated, A must pay the tutor his fee for the whole year. A, however, may bring another pupil to the teachr to be taught in place of his son.
SOURCES: Pr. 434–435. Cf. Agudah B. M. 118.

a) Q. A was a Gabbai (officer) of a cemetery for twelve years. He lent two marks to the charity chest of the cemetery. When he took an oath as to his wealth from which he was to pay taxes, he included among his assets the two marks due him from the charity chest, and paid taxes for the whole amount. Subsequently he resigned his post as Gabbai and demanded that the community order the new Gabbai to pay him the two marks. When the case appeared at court, A said to the community representatives: "I do not ask anything from you." The judges construed this statement to mean that A relinquished his claim, in spite of A's protests that he meant to say that he demanded the money from the charity chest and not from the community.
A. It is customary for charity officers to advance their own money when the charity chest is empty and to collect it afterwards. Such officers' claims that money is due them from the chest are believed. Therefore, A is entitled to collect his two marks, and his subsequent explanation of his statement "I do not ask anything of you", should be accepted.
b) Q. Some members of the community are ready to testify in A's behalf. Is their testimony admissible as evidence?
A. The testimony of residents of a city is admitted as evidence in a case to which all the residents of the city are a party, if their testimony is against the interests of the city residents. Such testimony, however, is not admissible against the relatives of the witnesses.
SOURCES: Pr. 1012; Mordecai Hagadol, p. 301a; ibid. p. 350a; cf. Mord. B. B. 489; Weil, Responsa 124.