At the beginning of the post, I thought it was strange that this guy offers you to pay for a drink later, because he could offer you a deposit option... And voila, you yourself suggested it :)

I'm not sure that this can be called microinvestments, because this is a really proven option for a deposit with a supplier. But, if over time, you get the 5th bottle for free or at a discount, then yes, it already looks like an investment ;)