One has a moral duty to keep one's promises.

Scanlon claims that promissory obligations derive from another sort of more basic moral obligations, specifically obligations not to “unfairly manipulate” others. One has a moral duty to keep one’s promises because making a promise will lead others to believe that you will do what you promise. Breaking the promise is then tantamount to deceiving those one promised, and since one has a moral duty not to do this, one has a moral duty to keep one’s promises.