Accounts receivable: items interest calculation

Hi,

I configured a demo of accounts receivable items interest calculation. However, I have the following issue:

Ex:

-Customer 150

-Interest calc. indicator Z1 --> Payment terms ZIN1 (5%)

-Interest calc. indicator Z2 --> Payment terms ZIN2 (7,5%)

We have different types of revenues for a single customer. We would like to be able to caculate different types of interest for the same customer.

Ex:

Document number 1: 500$, payment terms = ZIN1

Document number 2: 1000$, payment terms = ZIN2

So, when I run t-code FINT, the system should calculate an interest of 5% on document number 1, and an interest of 7,5% on document number 2.

However, the system does not react like this. It seems that SAP always look in the master data of the customer to verify if the interest indicator is defined in the customer account. That means what we can only set one interest cal. indicator at a time in the customer account, i.e. we cannot calculate multiple interests for the same customer.

Is there any way to do so? How come SAP is coded ike this??

Thanks a lot,

Julien