I doubt that she will find it worth it to sue for 300 dollars. You clearly have a verbal confirmation that she wanted the spots and she did provide the payment. assuming your contract states that the deposit is non refundable i wouldnt give it back. She hasnt even paid the whole deposit. she needs to understand that you will find it very hard if not impossible to find anyone to fill summer spots at this time. I wouldnt cave to her threats. think of what its costs us to sue when parents dont pay. Its really not worth it.