Board Thread:General Discussion/@comment-27524978-20160210030742/@comment-5815618-20160505173725

Sans has a SOUL, that is known for sure (we see him as one of Lost Souls, etc.), and Asriel expels all SOULs he absorbed at the end of the fight. Had Asriel kept Sans's SOUL, he wouldn't be alive since Sans can't have a body in the overworld if Asriel has his SOUL (whether it is human or monster, it doesn't matter since the body is the product of the SOUL).