While more common a couple if years ago, sellers would have to pay (instead of getting paid) when escrow closed. While they may be able to pay they might not be be happy about it.
If not, than you may be looking at a short sale situation. This is, the seller must have their lender approve of the sale and agree to accept less than the full balance of the outstanding loan.
The seller and their agent would begin discussions with the lender.
Hope this helps!
If the lender doesn't allow the short pay off by the seller then the ratified offer is null and void and the bank forecloses. You can come back at it then if the opportunity presents itself.