Ah, I think I misunderstood your question.
Credit card interest is charged on the daily cleared balance. If you pay your full bill in time, it's interest free for the purchases that month. However, if you don't pay it in full, the interest is charged from the date of transaction to the date you do pay it. So if you pay 3900 off a 4k bill, in the first month you are charged interest on the full 4k. But the next month it's only charged on the 100 remaining.
That's how all the banks work.