well, u really cant do anything, because the sale is subject to lender approval. they need to approve all liens, before they go with the deal...
so bottom line, figure out what kinda of lien this is, if its a tax type irs lien, or hoa lien? it has to be resolved because now they are figuring out if the bank has to take that 15K hit, cause the seller of the house probably doesnt have the money to pay. they might have the seller do a promissory note if the bank fronts that 15k.