“Trump signed an agreement to do something, now he’s backing off of that agreement. Do you want this man as president?”
A valid agreement always requires reciprocal consideration, and the RNC appears to be reneging on its commitment to deliver upon its obligation for that agreed upon consideration. An RNC failure to perform in accordance with the agreement effectively invalidates the agreement due to the RNC to deliver the consideration. Trump is not obligated to perform an agreement that is already invalidated by an RNC refusal to deliver the consideration promised by the RNC.
I would imagine that the contract Trump signed is pretty close to what the other candidates signed. Could you post any copy of that contract you might have?