Responsa על בבא בתרא 297:1

Q. A's father, before his death, deposited his books with B (a trustee) as security for a dowry of twenty-four marks, which he promised his son C. He subsequently paid B eighteen marks, leaving a balance of six marks. C's brother, A, representing his mother, Bruna, now demands that B permit him to redeem his father's books for six marks. B, however, claims that when C demanded the six marks from his mother, she, in payment of that amount, assigned to C a debt of twenty-three pounds D owed her, and pledged the books for that amount. B, therefore, demands twenty-three pounds to be paid to C before he returns the books. A, however, claims that anything paid to C above six marks will constitute usury.
A. B's testimony is accepted without reservation since B is a trustee. According to that testimony, the payment of twenty-three pounds to C would constitute usury. B must, therefore, return the books to A and his mother upon the receipt of six marks. Moreover, the loan of the value of twenty-three pounds never really belonged to C, since a loan can not be legally assigned to a third party.
R. Meir sent substantially the same answer to Rabbi Yedidyah which answer begins as follows: "According to the (record of) claims sent to me by the judges, the widow, Bruna, did not admit that the loan originally belonged to them (to B and C). She claims only to have given them the loan in exchange for the pledges …."
SOURCES: Pr. 985–6; L. 503.