Backward / Forward Scheduling doubts

Hello,

We have some doubts around the concept of backward/forward scheduling.

We have a given material with a forecast input in the MD61, lets say 500 pcs per month (march, april and may).

We raise a sales order for it with the qty of 1000 pcs for April, 30th.

If we confirm for "Delivery proposal" on the availability check, in MD73 the system occupies correctly the planning slots of march and april.

But if we choose the "complete delivery" button, the necessity is assigned to the may slot and leaves 500 pcs to be confirmed.

In other words, during availability check, if we choose delivery proposal, the system carries out backward scheduling. Otherwise, it carries out the forward scheduling.

Our intent is to have backwards scheduling always.

Any ideas of how to solve it?

Adriano Cardoso