Responsa על בבא קמא 226:3

Q. A, B, C, and D, were partners in a loan made by them. In repayment they received a quantity of silver which they divided among themselves by lot. Subsequently, A bought B's silver and sold it to merchants [probably Gentile merchants]. The latter broke up the silver and found it mixed with base metal. A averted a calamity by pacifying the merchants with gifts of money, thus preventing their bringing false accusations against him. A demands that B reimburse him with the price of the silver, and also compensate him for the money he had spent in pacifying the merchants.
A. The sale of the silver to A is void, since it was made in error. Similarly, the division of the silver among the partners is void, even though made by lot, since that too was made in error. However, B is not required to compensate A for the money he spent in pacifying the merchants, since B did not know, at the time of the sale, that his silver contained base metal. Moreover, even if B knew the contents of his silver, he would still be absolved from paying A the money he had given to the merchants, since he was only an indirect cause of A's loss, though he would be liable to punishment by the Heavenly Court.
SOURCES: P. 48, 49.

Q. A owes money to B and B owes the same amount to a Gentile. B told A, in the presence of the Gentile, to pay his debt directly to the Gentile. The Gentile released B and depended upon A for payment. A successfully avoids payment to the Gentile because of an old debt due him from the latter. Since, according to Jewish law, the transaction was not valid and A's indebtedness to B was not legally transferred to the Gentile, A must pay his debt to B. But why should B be permitted to rob the Gentile? Is not robbing a Gentile prohibited? Moreover, the Gentile presses A for payment and eventually may force A to pay him his money; must A pay his debt to B?
A. Robbing a Gentile is prohibited, but one is permitted to annul a Gentile's debt (if he can do so by using plausible excuses and without causing the name of the Lord to be profaned). But, if A will be forced to pay the debt to the Gentile, he will not have to pay anything to B.
SOURCES: Cr. 227; Pr. 327; L. 385; Am II, 119.

Q. A owes money to B and B owes the same amount to a Gentile. B told A, in the presence of the Gentile, to pay his debt directly to the Gentile. The Gentile released B and depended upon A for payment. A successfully avoids payment to the Gentile because of an old debt due him from the latter. Since, according to Jewish law, the transaction was not valid and A's indebtedness to B was not legally transferred to the Gentile, A must pay his debt to B. But why should B be permitted to rob the Gentile? Is not robbing a Gentile prohibited? Moreover, the Gentile presses A for payment and eventually may force A to pay him his money; must A pay his debt to B?
A. Robbing a Gentile is prohibited, but one is permitted to annul a Gentile's debt (if he can do so by using plausible excuses and without causing the name of the Lord to be profaned). But, if A will be forced to pay the debt to the Gentile, he will not have to pay anything to B.
SOURCES: Cr. 227; Pr. 327; L. 385; Am II, 119.