Don't pay for something you don't like

Our pharmacy deals with generics post-dispensing. As in we inform the customer about generics after we've dispensed it. If they want to the original brand, they can tell us when they hand the script in or after we dispense it.

We did this with an antibiotic. Dispensed another brand, asked the customer if it was ok.

Him: You dispensed it already. I'll take it but next time you should ask me first.

And angrily walks away.

Why would you walk away with a brand you don't want and then pay for it?

If you didn't like it, just tell us and we will change the brand back. Don't make it sound like we forced this upon you and get all upset and then go to the counter and pay for it. After all its your money. If a restaurant gave me a latte instead of a mocha I ordered, I'm not gonna pay for it. I'm gonna ask them to give me my mocha and then I'm gonna pay for it. 

He could've said no I don't want this brand. Simple as that. No need to make a big massive speech about RESPECT.