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 ;)
You are right, in the sense that an investment should have a clearly linked benefit, ON TOP of the returned money.
I personally think that in this case, the benefit is creating and implementing the habit, because this compounds and creates that sustainability, where you ride the wave of those short ripples you create.
In such a contest, this is undoubtedly a good investment ;)