The Collaborative International Dictionary of English v.0.48:

Promisor \Prom"is*or\, n. (Law)
   One who engages or undertakes; a promiser. --Burrill.
   [1913 Webster]

WordNet (r) 3.0 (2006):

promisor
    n 1: a person who makes a promise [syn: promiser, promisor]