Practical issue in SAP FICA Interest Calculations

I have the following practical issue in FICA Interest Calculations:

We are applying the interest key (through inbound interface) on line-Item level due to transaction specific legal requirements…. 

For example:

Let say Contract Account: 10001 is having following two receivables transactions

1.)   Doc.Type:B3, Due amount: $10,000.00, Due date: Sept 30^th 2012, Interest Key: C1 (6%)

2.)   Doc.Type:K9, Due amount: $5,000.00, Due date: Oct 15^th 2012, Interest Key: C2 (1 %)

When we execute interest at the month end on Oct 31^st 2012:

3.)   Doc.Type: IN, Interest amount: $53.13, Due date: Oct 31^st 2012, Interest Key: ??

My issues in above interest document are as follows:

1.)   Interest Key is not carry forwarding to interest document which was created after interest run.

2.)   Interest amount $53.13 is belongs to Doc.type B3: $50.94 and Doc.Type K9: $2.19

Issue 2, I may solve by executing separate batch process for each doc.type in order to post respective interest document, but how do I get the interest key on Interest document? Is there any standard functionality or do I need to go for event enhancement?

Thanks in advance guys…

Chinna
Katireddy@gmail.com