stipulator

stĭpŭlātor, ōris, m. [stipulor], jurid. t. t.,

one who demands a formal promise or covenant (opp. promissor, the one who gives the promise); a bargainer , stipulator , Dig. 2, 10, 3; 45, 1, 41 sq.; 45, 38, 5; Suet. Vit. 14; Isid. Orig. 10, 5.