Is it legal for my boss to keep my whole paycheck since he paid a bond that I was re-paying him for?
Question Details: I had a $850 bail and my boss bailed me out. Then I quit the job owing some on it. Can he just keep my whole check?
No, he can't.
First, unless you had agreed to repay him before he bailed you out, you have no legal obligation to repay (you may have a moral obligation, but that's between you and your conscience): without an agreement in place, his bailing you out was voluntary on his part, and when we voluntarily do something for another person, we can't obligate them to repay us.
Second, even if there was an agreement to repay and so you did owe your boss money, the law is very clear that an employer cannot withhold employee pay without employee consent: you'd have to agree to let him do this.
You could sue him (such as in small claims court, as your own attorney or "pro se") for the money. He could countersue or counterclaim or otherwise interpose as an "offset" any amounts you owe him--that is, if you had an agreement to repay, he has a valid claim for whatever part or amount you failed to pay. The court would then balance what he owes you vs. what you owe him and award the appropriate amount to one of you. (E.g. if he owes you $850 and you still owe him $500, netting those amounts out against each other, you'd get $350.)