Where a person (“the creditor”) has a contractual claim upon another (“the debtor”), and a third person has a duty to satisfy the creditor, or has in fact satisfied the creditor in discharge of that duty, the law which governs the third person’s duty to satisfy the creditor shall determine whether the third person is entitled to exercise against the debtor the rights which the creditor had against the debtor under the law governing their relationship and, if so, whether he may do so in full or only to a limited extent.