promisor

noun

1. One who engages or undertakes; a promiser.

   “Each of two or more joint promisors may compel every other joint promisor to contribute equally with himself to the performance of the promise, unless a contrary intention appears from the contract.”

   “[…]the creditor may request the promisor to perform his obligation to a third party.”

   “This may be attributable to a personal mistake of the promisor or negligence of her estate planner.”