When the new owners bought the business from your in-laws -- Did they keep the restaurants original name ? The buyers bought XYZ restaurant from your in-laws. Don't the new owners assume any liabilities and/or assets that the previous owners accumulated with the business ?
Suppose there was a contract for XYZ restaurant to rent items from a restaurant supply business - expensive kitchen equipment / dishwasher machine / huge gas grill + range + vent hood - and the contract had 6 months left on it between your in-laws and the Kitchen Supply Inc. The new owner assumes the existing rental contract , unless the sales contract specifically denotes otherwise. The new owners can't just tell Kitchen Supply Inc to pound sand, as they have assumed the liabilities of XYZ restaurant. Same thing goes for the assets, if it turns out all furnishings were included in the sale, and there is a million $$ painting on the wall, doesn't the new owner get to sell the painting or leave it hanging as they please ?
I'm potentially completely wrong here, but what I am trying to impart is that the new owner may believe that this refund from the phone company is an "asset" owned by the Business, not an asset owned by Mr. Jeromedawg's father-in-law. If my suspicion is correct, I think the in-laws should just forget about the $100 check, and not get into a disagreement with new owners , who owe many thousands more $$ still to be paid to in-laws.