In my opinion the search and the foreclosure are separate matters. Payment for the abstract is the liability of the party that hired you. The foreclosure is the liability of the home owner to the lender. The party that hired you is responsible for paying you. If they used your search for purposes of the foreclosure they have in effect rendered themselselves liable for payment...unjust enrichment....contract breach...in some states theft of services. If they used your abstract they have admitted that they found no problem with your work. 

It is a good idea to type a notation on your abstract that it remains your property until you are paid in full, and that their use of your abstract is expressly conditioned upon your receipt of payment in full.