Hi
In the definition of the rebate agreement VB(2, you can define the payment procedure, example
C no limits for manual payment. The option you choose there determines what you see at setlement or partial payment.
Sorry the Bo01 I was referring to is a standard rebate condition type defined in transaction V/06
I actually meant to use the most common one Bo03 (customer rebate) I have screen shots here
to show the case I was talking about
The example has a sales volume posted of 14.20 Eur the same amount was posted in accruals
but then I posted -4900 EUR manual accruals. I carried out the final settlement and the proposed
value in the sales document created was 14.20 (correct based on the sales volume) and the statistical condition is for 4904.20- (consuming all the open accruals)
See below: 
Hope it helps
Brian