Nonsense Meglet. Offer plus acceptance constitutes acceptance. Whether it's a test message or a handshake we have a contract. An offer can be withdrawn before acceptance but acceptance cannot be withdrawn after a better offer.
If you breach the original contract the offeror can sue to either enforce the contract, or damages. The offeror is however obliged to minimise his/her losses.
Forgetting the law for a second though: Don't you feel that you should be morally obliged to stand by the original agreement? Do the honest and decent thing!