Some great lessons here. OP, If it were me, I'd make one more effort at explaining the situation to a different person at the company. Lay out your case, explain you will now have to return the favor and that by protesting paying the bill any further would have possibly outed you as a shopper. I have my suspicions about this situation and ethical standards of the companies, but you did the work, and the restaurant was paid by a patron. It should not matter who paid.
I've had situations come up where the scheduler has told me to 'do the report and explain the situation' and I've been wary of doing a report if unsure about payment. It has worked out but after reading your case, I know I will never do a report if the reimbursement is in question.
More than twenty years in the business, and I'm still learning lessons. Thank you OP. I wish you luck.
Evaluating and mailing packages since 1994. I am an undercover connoisseur of customer service, a master of disguise in the aisles, and a sworn enemy of subpar experiences. I blend in, observe, and report—because excellence should never be a mystery.