SD - Retail functions on R/3

Question

I am trying to map some function which are Retail Specific in an R/3 Implementation . I need suggestions for approach to these 2 specific scenerios .
1. How do I handle initial Cash payment for purchase made , It can be partial payment , full payment , any % tage of payment due.Like SD order the billing Document is generated after Billing but we need a mechasim to handle the intial payment.
2. Second part of Payment due is basically paid by company but Customer pays back that in installments with interest Charges , So Customer may pay 10% of intial payment but 90% is financed at 24% interest .
I am unable to think of functions in R/3 SD which even come near to the functions required.

Answer 1

Well I have looked into billing plan but these are the issues I am running into .
Billing Plan splits the Billing into multiple sets and Revenue dates are also defered based on that .
The Business works this way
1) After Down payment is made - The rest of the Due is financed and they want the installment splits to be made at this stage .
2) After Delivery the Revenue has to be recognised 100%.
3) The due now goes into A/R cycle .The Customer is billed along with the IS-CCS Billing for net due every month.
4) Customer should be able to pay off their prinicple any time and hence reduce the interest payment .
I am not even able to figure out if we built a custom function how do we integrate it.