What is the law regarding signed contracts after the service was performed?

I am selling my house. On October 28th, the buyers inspection period ended. On this day, I am informed that they want me to make the repairs. These were minor repairs that did not require a license. So I hired a guy and got them done right away. After I paid the guy and the repairs were finished, I am sent an addendum on October 31st. This addendum states that seller agrees to hire licensed and insured contractor to make repairs. The contractor told me he was licensed so I signed it. Later, the buyer finds out that he actually wasn't and wants to walk away. Did I shoot myself in the foot by signing that addendum or do I have a case considering there were no instructions on who I should hire before I hired and paid the contractor?