As other have already said in their own way - the lender who holds the current mortgage on a property being sold is going to come up "short" of what's really due to pay off the mortgage in full.  Therefore, the current lender must agree to the selling price & all terms of the sales contract.  Keep in mind that because of this additional cooperation from the current lender, the timeframe to complete this type of sale is typically anything BUT "short."  It will usually take several extra weeks to close on this type of purchase.