Laundering money through a fast food restaurant with a supply chain outside your control is a terrible idea. Franchises are required to purchase food centrally.
Q How much cash are you banking this week?
A 1,000,000 EUR
Q So you have sold 500,000 Big Macs.
A Yes
Q Can I see your invoices for 500,000 buns please?
The profit margin is not high enough for it to be worth ordering extra stock and throwing it out. The business would effectively be paying about 70% tax. That's not digestable, even to launder cash.
More plausible reasons: Running a McDonalds franchise and significantly and systematically under-reporting revenue using unauthorised suppliers. But to involve low-level staff would seem unlikely.
This isn't exactly covert surveillance. Some bored member of staff just didn't like the look of this American weirdo with a video camera for an eye, a piece of paper that he waves around and an attitude that he is entitled to buffer everything he sees anywhere in the world and then publicly blame an entire multinational for a minor, local incident. (Not my perception, but I think it likely that it was theirs.)