Phrase meaning whoever is buying can choose

I feel a simpler substitution would be "Your call"

From UsingEnglish,

If something is your call, it is up to you to make a decision on the matter.

Usage:

Friend: What will you take? Heineken or BudLight?

You: It's your call!

You pay the piper, you call the tune

He who pays the piper calls the tune.

Something that you say which means that the person who provides the money for something can decide how it should be done. Cambridge Idioms Dictionary

The person who provides the money for something has the right to determine how it's spent. ODO

You might just say "It's your money", thereby implying that it's his choice to make.

Dealer's choice in poker refers to the ability of the person who distributes the cards to specify variations on the rules of the game. For instance, the dealer may say "Deuces wild" and for that hand, 2's, ordinarily the lowest cards in the deck, become extremely valuable, because the holder of a 2 can designate it to be any other card.

Thus your friend was the dealer (distributing beer, not cards), and you were allowing him to make the rules about which beer to buy. So what you said was appropriate, but someone who has never played or heard about a card game with dealer's choice would be completely mystified.