My Ex Fiance Is Threatening To Take Me To Court If I Don't Pay His Mom $14,000

TLDR: Ex fiance’s mom gave us $13,000 cash to pay off debt and gave us a $5,000 wire transfer gift, after $11,000 was paid back she is now demanding $14,000 for interest and to back charge rent.

My ex emailed me this morning threatening to take me to court if I don’t pay her back. We had been together for 4 years and were already engaged when we started living with his mother in order to save money to buy a house. She gave us $13,000 cash in order to pay off various credit cards to improve our credit scores with a verbal agreement that we would be paying it back. My fiance was paying her $600 a month (for the last 18 ish months) to start paying her back while my money was going towards saving up for the house and various other things. She had also given us an additional $5,000 as a gift with “no repayment is expected or implied either in the form of cash or future services” to help towards the down payment on the house which was wire transferred to my fiance’s account.

When we bought the house $14,500 came from my bank account, $5,400 came from my 401K and nothing from his accounts. We broke up about a year after we moved into the house and it was very civil with verbal agreements that he would finish paying off what was owed to his mother, I could keep the house and he would continue making mortgage payments until I was able to find roommates. A few weeks later this changed and I was told that he would no longer be making payments towards the house that his mother wanted $11,000 from me for the initial cash she had given us to pay off debt. I started working 60 hours a week and his mother said that if I paid her back $10,000 that would be fine. I still wasn’t able to afford the house on my own so I recommended we just sell the house, which we did. It is officially sold and we split 50/50 with us each getting $8,500.

The day the paperwork was signed stating how much we were going to each be receiving she said that I actually owed her $12,500. According to her, this is because I had to pay for the $5,000 gift as well. At the time I didn’t realize that my ex had paid back roughly $11,000 and so I said I thought $7,000 would be fair because she did help us out a lot and I could pay that out of the house money and my ex could pay the rest. She said I would have to discuss that with her son, which I did and he said he’d cover whatever was left. After this, she called me back saying I actually owe her $14,000 because she was going to back charge me rent as well as interest on the initial money given. At that point I stopped responding to calls, texts or emails because I don’t feel comfortable giving her anything now because I believe that it will never end.

After doing the math and realizing he had already paid back nearly $11,000 (my money was going to various other bills) and she was now demanding $14,000 because of interest and back charging rent I don't know what to do. I have a letter she had sent to the mortgage company that she signed stating that we had been residing at her residence “rent free” for the year and a half before we moved into the house so I don’t think she can back charge the rent. The $5,000 I don’t think is my responsibility because 1. It went to my ex’s account directly and 2. I have the document for the wire transfer stating it as a gift. Can she force me to pay the full initial $13,000 though? He would make withdrawals from his account and was paying her cash so I don’t have any way to prove where that money was actually going. So because she can prove that I deposited the cash into my account and I can’t prove anything was paid back I’m nervous and not sure what to do. I live in Florida btw.

Any and all advice would be appreciated and I can give more information if it's needed. Thank you!