I had pretty much this exact thing happen to me. I had left one company to go work for a startup. The startup failed. I was contracted to come back to the first company to cover for a former teammate who was going on maternity leave.

For insurance they put a sizeable early leave monetary penalty into the contract. I thought just giving my word would be sufficient but I had no plans to leave so didn't matter either way.

During the contract I did well on an online programming contest that turned out to be a recruiting tool. I was made a now or never offer I shouldn't refuse but I did explaining that I was committed to my current contract. When discussed, they we're willing to payout the penalty to have me start earlier.

At this point I thought hard about my personal ethics on this and it came down to if it was based on trust, I would have stuck it out to the end but since they made it monetary I was playing by their set out rules and had no qualms (well maybe still a little) about going before the end.