Any ways to insert header condition into billing directly?

hi All,

Right now, our customer has a requirement to offer customer a discoust by certain percentage and basing on subtotal2 price (in process value) in billing.

But the funding of this discount has limit. Another word, we cannot set this discount by percentage, because it may exceed the limit for the last order.

So the best solution we can think of is to place a header condition (not item condition) into Billing. the question is HOW.

we knew, for sale order, we can call BAPI "BAPI_SALESORDER_CHANGE" to insert header condition into sale order. but we cannot find similar functions for billing. Are there anyone can find similar functions to place header condition into Billing header??

BTW, BDC is not recommemded by customer. please try another way.

Thanks.