To sue or not to sue?

What is the name of your state? Wisconsin

My fiance and I recently split. He used a lawyer's letter to dictate when he would move his things. I responded to the lawyer in writing with my own terms for the move which included any items left in my possession after that date would be considered abandoned and also asked the lawyer to advise how a loan would be for for which I was the cosigner with my boyfriend. The loan was a signature loan, but the money was used to buy my fiance a car. The lawyer responded to my terms indicating the my fiance agreed to the terms of my letter and agreed to pay the loan.

I received a call from the creditor indicating the loan was not paid on this month. So far, my ex-faince made only 2 of the 3 payments. I called a family member of my ex-fiance to get a message to him that the loan needed to be paid for. I then received a call from the lawyer stating there was a list of personal property in my possession including gifts of jewelry that needed to be returned before my ex-fiance would pay on the loan. According to the lawyer's letters from 4 months ago that I have, my ex-faince agreed to the terms of my letter.

Can he now demand more items returned (some of which I don't believe I have) including jewelry that was given to me and is there any possibility of proving that loan was used to purchase the car he has and if so can I put a lien against the car? I know for a fact he is exempt from garnishment due to the percentage taken from his pay for child support from his divorce.