Account assignment in PO and rebate provision condition

Hi,

We are starting a new procurement process in SAP and we are having difficulties with the account determination. We created a PO with account assignment. There we define the gl account and the cost center. In our condition we have some rebate condition that apply. The rebate condition has an account key (ZB2) and a provision key(B01) define. when we do a good receipt the result we get in term of accounting is:

Dr - Purchase account 100

Dr- Amt due by vendor account 10

Cr- Clearing account 110

What we wanted was:

Dr - Purchase account 100

Dr- Amt due by vendor account 10

Cr- Clearing account 100

Cr - Rebates provision account

We believe to have correctly configure the accounting key ZB2 but the standard is not taking it in consideration. One thing we noted is that if we do not do account assignment at PO level but through configuration we get our expected result. We wanted to know the reason behind this and the possibility of getting out expected result using account assignment at PO level. Thank you for your help.