The store I worked at had just gotten a new line of flat-pack furniture. The general manager had offered my $20/piece to come in on my day off and assemble the display models. There were 25 units, and I managed to build 13 of them in a day (it's pretty easy, once you get into a groove, and with no customers bothering you constantly). After I was done for the day, I went to the GM, “$260, please!”
At that point, he decided to just pay me my hourly rate (around $12/hour at the time) instead. He literally told me that he hadn't expected me to get so much done in a day, otherwise he wouldn't have offered me such a great deal. So, $260 cash became $96 before taxes. I was so pissed, but couldn't afford to quit at the time.
The next day, he told me to build the rest. Since it was a regular work day, I made sure to serve EVERY SINGLE CUSTOMER I could find. It took the rest of the week to finish the remaining 12 units. I also never came in for another off-the-clock cash-job for that manager ever again.