Or free labor. I notice they haven't talked of paying him to operate the bar. And just a few chapters ago they pointed out bartenders are paid peanuts.
It wouldn't be surprising if it turned out Uncle can go back and forth between the two worlds if he just asks the spirits. His character arc could be to realize he left women who loved him behind.
I don't really understand wanting to be a bartender either. I understand falling into it because you need a job, but who would want to quit a job to serve drinks?
And "delicious"? It's not like you even brew those drinks.
4 billion yen to save the whole kingdom sounds pretty cheap. Even if they're cash-poor, the king could have thrown in stuff like hereditary nobility or something, rather than the vague "you'll have preferential treatment".