Yes there are factors involved that can go wrong. One of them is not you backing out of the agreement because you feel like it. If both parties agree to it then that is different. Otherwise I can't believe we are having a discussion about honoring a commitment. Now I know why disagree on so many things.
A tells B, there's this potential deal that you MIGHT be able to get, if you're able to get it I will purchase it from you for $50 over your paid price as long as you deliver as instructed.
B agrees to the terms.
BEFORE the deal goes live, B learns that the profit offered by A is below market value.
B calls A and says I have learned that you got me to agree to a lowball offer, will you raise your offer to match the market.
A can say any of the following: 1. Yes, I will raise my offer, 2. No, but if I find out that you sold to someone else, you are in violation of our agreement. 3. No, because I have a great reputation, and always stick to my commitment, even if market goes south, the choice is yours - but remember that it is a two way street.
B has various choices, depending on what A says. One of those choices is to simply pass on the deal. I think most reasonable people would stick to their current commitment (unless they actually tried to renegotiate) but might do more shopping around in the future.