Is this correct? You have one lot with a house on it and another lot with no house? Yes? And you want to sell the empty lot?
No, you do not have to pay off the mortgage before placing the lot for sale. Once you receive an offer and come to terms with the buyer, the paperwork will be sent to a title company for closing. Your agent will take care of that. The title company will take care of dispersing all funds to the appropriate people and places at Closing. You will get whatever is left after all outstanding balances are paid.