I added random tests now, thanks for your help.

You are right, I wanted to add some random tests but haven't yet.
Is there a better way to access my solution to use it in the tests to calculate the result for the random input than copying it to the test code under a random name?

If the balance is negative, let's say -100 for 10 days and interest per year (assuming 365 for this reply) is 0.01 or 1%, then you would calculate -100 * 10 days * (0.01 / 365) = -0,027397260273973
So, yes it would be deducted, you would have to pay interest to the bank for that period.