I have this happen from time to time - order too much 87 so the excess goes into the 91 tank.
I mark it as 87, sell it as 87, and run the tank down as low as feasible, and then start over again with fresh 91. My loss, buyers gain. I don't sell 89. I normally don't keep much 91 on hand, it's not a big seller at my c store, so it's not much of a catastrophe when this happens.
The retail price of gasoline is so high now, the margins are tolerable without having to resort to fraud on purchasers. I am sure it still happens, but it probably happened a lot more when gas was a buck or less a gallon.
If you think you have purchased adulterated fuel, I would try to report it the brand if it is a major. They take this malfeasance seriously.