MacBook stuck in lost mode

I sold my 2014 MacBook Air to my friend, but he was behind on payment so I locked it until he payed me. When he made his last payment we called, but I forgot the code so I asked him to try different codes. They didn’t work. I can’t remember the code, and I can’t go on Find My, since I already took the MacBook out of my iCloud. What can I do?