My former roommate and I leased an apartment together last year. After the lease ended, a debt collection company called me saying that I owe money to the apartment company, asking me to pay for the debt. They also called my roommate but he never answered their calls. Then I contacted him and he agreed to pay for the debt together, and he agreed that he would pay to the debt collector so I transferred my part of debt to him. But he never paid! I asked him to give my money back. But he never answered me. Now he left the country and the debt collector sent me a debt collection letter to ask for the total amount of that money. And they even put a potentially negative item on my credit report. In this case, is it possible to sue my roommate even though he already left the country? Or what shall I do to only pay for my part and avoid the negative credit report?