@Daljit SinghI would just remove the tank.Empty it of fuel.Force air into it and vent from the top until the tank is dry.You could fill it with TSP and water, air dry it again, and cut it up with a Sawzall for scrap.Or sure, you can get a professional involved that could run you some big bucks.Fuel tanks don't scare me.

@Jim K.Its a pretty dry climate so the bones are usually good, the studs are usually better then the modern ones!

Tile over that when the membrane dries using a good modified mortar -- looking at Home Depot's line of Custom mortars, I would stay away from something like VersaBond here and go with the more expensive FlexBond for this application.

This should still be a good deal for the buyer, since housing prices likely went up more than $30K since they signed the contract.I just think as a matter of integrity, trying to honor your contract and not leaving the buyer high and dry is the best choice for your business. 

Lest we forget the drying up completely of investor mortgages in late 08 to 2010 or so caused one hell of a mess to our economy   

But ,we're not going to leave you high and dry!

and if needed Kilz on top of that so want it to be dry. 

If vacant you need to:what is cost of dumpsters vs buying a pick up or finding some hauler to throw away all the demolition  secure and board up and add cameras, get roof bids, order roof parts, roof needs to get on the house ASAPclean out dry wall, everything fabric throw away, cut up and remove carpet, mop the studs and floor with bleach and water solution (5% bleach), inspect electrical - maybe some shorts or problems, order flooring for kitchen and baths and new kitchen cabinets (measure like you are OCD ten times), how is the sub flooring?

Orange peel can dry out and crack over time.

Off market is pretty bone dry right now as well.