I do freelance embedded systems development. I do fixed bid work because I prefer it that way.
If I bid $2,000 to produce something, I either produce it for $2,000 or I say (hopefully within the first day) "sorry, I underbid this. I need to re-quote you, or you can have your down payment back." I've "eaten" quite a few underbids and only once returned a down payment and ended the job because the buyer kept increasing the scope.
OP's not "abusing a fixed price worker," he's holding them to what they agreed to do.
If I bid $2,000 to produce something, I either produce it for $2,000 or I say (hopefully within the first day) "sorry, I underbid this. I need to re-quote you, or you can have your down payment back." I've "eaten" quite a few underbids and only once returned a down payment and ended the job because the buyer kept increasing the scope.
OP's not "abusing a fixed price worker," he's holding them to what they agreed to do.