The way I understood your parent comment:

- The vendor charges every customer according to the customer's social class. A white guy pays through the nose. A servant-class local pays a modest amount. It doesn't matter who the food is supposed to be for.

- Thus, it is cheaper for the white guy to hire a housekeeper than to buy and prepare his own food. The white guy's food, if he has a housekeeper, will only cost as much as the housekeeper has to pay for food, because the housekeeper is responsible for buying it. And the savings relative to the white-guy price cover the housekeeper's wages.

The poster got the OP’s point but you didn’t get his. Presumably the expat is giving the housekeeper a grocery allowance, she’s not spending her own money. The merchant could certainly make a deal with the housekeeper to charge expat prices and the two could split the difference.

That not being the case is the system that got the housekeeper her job though. It's probable that she knows that "not paying the foreigner tax on food" is part of the value she brings, and that overly messing with that system could put her out a job.

That only works if the foreigner has no friends and doesn't talk to anybody about how much they pay their housekeeper and the grocery budget. And if the foreigner just pays t housekeeper a lump sum, rather than a wage + cost of supplies, then that also won't work.