I had an agreement to pay my job back in writing.my job still fired me what can I do about this?

Presumably you agreed to repay your employer becuase you owed it money.  Did your repayment agreement also provide that, if you repaid your debt, your employer would not fire you?  If so, your employer has breached the agreement, and you can sue it for breach of contract.  If not, however, I don't think you have a valid claim, unless you believe that you were terminated for other impermissible reasons, for example your race, gender, religion, etc., or in breach of some other agreement like your employment contract (if, for example, it provided that you could only be fired for specific reasons, or that you would be employed for a specified period of time.