I think we should get interest for the tu we put in the bank.
I don't know how much I have to clarify about what it means, but an idea for that the more tu you have in your bank account, the more interest you get per week.
I dunno, should it be week/day/month?
I think week sounds reasonable, while day seems like too much. Month seems far too long for me, but please do comment if you think month is better.
Feel free to elaborate on any specific ideas you have!