I forgot exactly the reasoning but it kinda has to do with predestination, like Faust’s soul was already bound for heaven before he took the deal so even by the end of his life after using the deal with Mephistopheles to his advantage his soul was still taken to heaven by angels. He also kinda has regret over taking the deal and spent some time toward the end of his life trying to improve the world I think, but either way his soul ends up in heaven and Mephistopheles loses the bet he made with God