I worked at a job early in my career. To be fair, it was a good job and the boss/owner was decent. I got a great opportunity that paid $25K more per year (or about $2,000 more per month). So, I gave my two weeks' notice. Boss was shocked and demanded I give at least one month's notice. My new job would have been fine with that, but it would have cost me the pay difference for those two weeks (a little over $1,000 gross). I told my old boss I would stay if he would make up the salary difference for the notice period.
He looked at me like I had three heads. The idea of screwing him over was offensive to him, but the idea of screwing me out of money totally and completely made sense to him.