My brain immediately suggested the non-word "promisand", but I doubt I would be understood if I said that. What's a good word (or failing that, phrase) for the action or thing that was promised? This reminds me of Latin expressions like Carthaginem esse delendam (Carthage must be destroyed, lit., 'Carthage is a thing-which-is-to-be-destroyed') or perhaps that once-common mathematical expression QED quod erat demonstrandum ('that was a thing-which-is-to-be-proven').
Circumlocution or other forms are often possible: "He gave me the promised widget" or "He did what he promised he would". But sometimes it's useful to have a word that stands on its own.
More generally, is there a good way to express the construction 'thing-which-is-to-be-X'? In Latin this is a gerundive, though it seems in my brief searches that the term means different things in English and other languages.