Very mediaeval... and not in a good way.

It has a good message... kind of, but that's not how God works, sometimes. If I were a new Christian, or someone learning about the faith, I might read this and think of God as a genie. God works in mysterious ways, but He doesn't always reward like that.

Sure, Billy was honest, but by reading this, I might also think that God rewards those who return money that randomly happens to belong to a spiritual leader. The theme of the story is generally good (honesty is rewarded), but it's not portrayed very well. Sorry.