If this went in front of a judge, the judge will demand answers from both of you as to what is going on here? Why would someone GIVE you a check and then only want some if it back? It sounds suspicious and there'd better be answers.
It sounds like the classic scam where someone overpays you for something you're selling, by check, and wants you to give them back the amount of the overpayment. So you do. Then their check bounces and you lose.
So, what is the reason for her giving you this check to cash? If you have taken it to your bank, give it fourteen BUSINESS days to make sure the check is good. If it is good, then keep only what she stated you could keep and give her back the rest. Because if you don't, you can end up in court, and if the judge doesn't like your reasoning for why you get to keep all of it, you may be ordered to give back everything and pay her court costs.
This sounds shady as hell....