I have a very short argument to the effect that the promising-relevant pro tanto moral obligation is not a pro tanto moral obligation to keep one's promises but, rather, a pro tanto moral obligation to not break one's promises.
[The argument crucially depends on the notion of a conditional release from a promise. Whereas an unconditional release (or a simple release) is a promisee's saying to a promisor something like "You don't have to phi", where phi-ing is something the promisor had promised the promisee to do, a conditional release is a promisee's saying to a promisor something like "If C, then you don't have to phi". I take it that both simple and conditional releases are altogether ordinary moral phenomena.]
Here's the argument: Suppose that on Monday A says to B "I promise to lend you my recording of Don Giovanni on Wednesday" and on Tuesday, B says to A "If you let me hold onto this copy of Cosi fan Tutte which you lent me the other day and were planning to take back today, then you don't have to lend me your Don Giovanni tomorrow." In this case, it seems that the following two courses of action are both permissible: (1) A takes back his Cosi fan Tutte on Tuesday and on Wednesday lends B his Don Giovanni, and (2) A does not take back his Cosi fan Tutte and on Wednesday does not lend B his Don Giovanni. However, option (1) involves the keeping of a promise and the breaking of no promises and option (2) involves neither the keeping nor the breaking of any promises. If there were a pro tanto moral obligation to keep a promise, as opposed to merely not breaking one, in this case (1) should be obligatory and (2) impermissible. If, on the other hand, the only promising-relevant pro tanto moral obligation were to not break one's promises, then, as neither involves the breaking of a promise, both courses of action would be permissible, which, I contend, is in fact the case.