That which was promised

Hmm... pledge or promise can work in some contexts, but not all. I also don't know a good answer to the more general "thing-which-is-to-be-X" case.

I don't think there is an answer to either your specific or your general question in English.

The closest is the phrase "what was promised". But note that there are plenty of other expressions which can be expressed in a word in a synthetic language like Latin but need a phrase in English: many of them involve participles.

So for example "loquens" might be translated "speaking", or "who is/was speaking" or "a/the speaker", or "one who speaks" depending on context.