No idea where it's from, but the quote likely is a less cryptic variation of "foolish consistency is the hobgoblin of little minds." (A related and more cynical French variation is that "promises only commit those who listen to them".)
That is, he means to raise awareness that a promise, belief, opinion, statement, etc. is only valid within a certain context. Should the context change, the promise, belief, opinion, statement, etc. may continue to hold, or it may very well fly out the window.
Imagine for instance that you're offered a quote for something at $X. You say "Yes!" But then, a few moments later, all sorts of hidden costs get thrown in - a murky sales technique called low balling. Many people will unfortunately keep their earlier commitment; the quote is a reminder that you can reverse it - since the context changed - and shouldn't feel sorry for whoever you promised a sale to.