Responsa for Ketubot 169:5

Q. May the sons of A's second wife plead that A's first wife, at A's request, has waived her children's right to her ketubah? Thus, while a person may not claim to have repaid a debt before it fell due because the presumption is that people generally do not repay a debt before it falls due, no such presumption exists in regard to waiving a right or an obligation.
A. The claim that an olbigation has been waived in one's favor is weak and is valid only when the legal presumption of Miggo exists; as in the case where the person could have successfully denied the very existence of the obligation, or could have claimed to have already discharged such obligation. Therefore, the claim in our case is invalid. Moreover, had A's first wife actually waived such obligation in the presence of witnesses, her act would have been invalid since she had no right directly to waive the monetary rights of her children, while retaining her own rights.
Q. Since the court does not attend to claims against young orphans except when interest is accumulating on their obligations, or when the widow's ketubah is demanded, in which latter case the court attends only because of hinnah (in order to enhance happy marriages); and since in our case (where A's sons by his first wife demand the ketubat benin dichrin) the element of hinnah is absent, must these sons wait (with the collection of their mother's ketubah) until A's youngest sons grow up?
A. A's sons by his first wife do not have to collect the ketubah of their mother; for it fell to them as an inheritance and is theirs already. All of A's sons are partners in the estate, which must be divided among them to give each of them his due share. Therefore, if A left money, the sons of his first wife may take their share of the money without the supervision of a court since the dividing of money between heirs needs no such supervision. But, if A's estate consists of real property, the sons of his first wife will have to wait until A's youngest sons grow up, as R. Tam is of the opinion that a court should not attend to the division of such an estate unless the young orphans agree to the division.
SOURCES: Am II, 65.

Q. A claims to have pawned with B, for thirty schillings, objects worth ten pounds. He is now willing to repay the thirty schillings and demands that B return the pawned objects. B, on the other hand, claims to have followed A's instructions and to have pawned the objects with the Cahorsin. To A's charge that the objects are still in B's possession, B retorts that he subsequently bought them from the Cahorsin. A also claims that B gave false testimony against him and caused him a loss of eleven pounds. B denies this charge. A further claims that he has provided B's Pentateuch with punctuation marks, that he was not paid for his labor, and that he can prove through witnesses that B owes him three pounds. B, however, claims to have already paid his obligations to A.
A. Regarding B's claim that he had paid his obligations to A, if he will take an oath to this effect, he will be free from obligation to A. Regarding A's first claim, however, the following distinction must be made: If B admits that A had originally pawned the objects with him for thirty schillings, even though subsequently thereto A told him to pawn them with the Gentile, he must return the objects to A upon the receipt of thirty schillings; but, if both A and B admit that originally A merely sent B to pawn the objects with the Gentile, B must state under oath the amount he paid to the Gentile for the return of the objects, and must return them to A upon the receipt of such amount.
SOURCES: B. p. 292, no. 370.