late payment fee assistance

Hi guys,

I have this basic late payment template. I'm trying to figure out how I can implement a method that will alter the applicable fee depending on if it is smaller or equal to the payment received.

If the payment received is smaller than the applicable fee, then the remainder should be added to the principal and interest penalties should accrue on that amount.

Also if the payment is received and not in full, then the days overdue should remain the same and not go down.

Any help would be much appreciated!

Or any input on better methodologies / approaches to creating a better template.