When you put a stop payment on a check...?

My roommate had to move away about 6 months ago due to health reasons and I found a person to replace her on the lease (without a penalty) and we all signed a contract stating that we would take care of the deposit outside the property management company's responsibility. I had given her 2 checks for her portion of the deposit (security and pet) in good faith before she moved. She did not fulfill her part in the move out expectancies-damaged carpet from her dog, cleaning her room out, leaving belongings for me to bring to the dump, or contributions for the final bills. I put a stop payment on the second check, as I felt the cost in time and the eventual deductions from my deposit warranted this. She has since given the check to a collections agency which contacted me. What are my legal options in a case like this, anyone know?