Justin informs Joseph that he will not be able to accept the contract for doing woodwork for the winter at Joseph's farm because of prior commitments. A day later, Justin calls up Joseph and tells him the original terms of the contract are acceptable to him and he can take up the offer. Joseph tells Justin he would indeed like Justin to do the work; however, her will pay only $$15$ per hour rather than $$18$ per hour as mentioned in the original contract. This proposal made by Joseph illustrates a Blank$\_\_\_\_\_\_.$