Promises should not be kept if they are disadvantageous to those to whom you have made them

Cicero, On Duties, Book I, p.13.

Nor, if they harm you more than they benefit the person who you have promised, is it contrary to good to prefer the greater good to the lesser. For example, if you had made an appointment to appear for someone as advocate in the near future, and in the meantime your son had fallen seriously ill, it would not be contrary to your duty not to do as you had said. Rather, the person to whom you had made the promise would be failing in his duty if he complained that he had been abandoned. Again, who does not see that if someone is forced to make a promise through fear, or deceived into it by trickery, the promise ought not stand?

Cicero is suggesting that the unethical behavior of others may relieve us of the obligation to fulfill out commitments. Where does this sort of reasoning end?