Can my lawyer adjust the amount that I’m entitled to receive after I’ve agreed to a certain amount?

I accepted and signed the papers for about 5,300 on a car accident settlement. Later I got a call from someone in the office saying that I would receive 5,900, and then later that day she called again to say that I would receive 5,200 because something wasn’t applied correctly. Are they allowed to do this after I have agreed to a certain sum of money?