If it's a chattel loan (personal) you will no doubt have to pay it off as I've never heard of an assumable chattel. If it's in a resident owned development there are very few conventional loans on RE that are assumable nowadays. Also if the home was built before June 15, 1976 I'd stay completely away from it.
Unless the loan is an assumable loan it will need to be paid off before escrow can close. If it is selling for less than the amount owned on the home, then someone will either have to pay off the amount or get the lender to take a short sale.