Bill-back rebate

Dear community,

We have bill-back rebate type deals with suppliers in which there is a discount (%) based on the net purchases.

Net purchases are calculated from purchases minus products returns made during the period.

On a regular basis, all returns are discounted even though returned items have been purchased in previous periods.

Normally, these are long-term negotiations and the income is billed to the supplier on monthly or weekly periods. The income calculation of this type of deals in RMS only reduces returned items as long as they have purchases of those items during the reporting period.

Does anyone else handle this kind of deals? How have you managed this situation?