I owe you one!

I owe you one! (used when someone has done something for you and you'd be happy to return the favour one day) — С меня причитается! Я (теперь) твой должник!

Example: Thanks for helping me out, Dan. I owe you one!
Example: I owed Dan big time for giving me his tickets.

- an informal way of saying I owе him a lot, a great deal; I very much appreciate what he's done

see also
[big time]