G/L account no in purchase requisition/order

Dear all,

I have a little problem. During the creation of a purchase order a wrong G/L account is automatically assigned. With the account assignment category "Q", SAP determines automatically the G/L account 30000 and disables this field. However the account 60000 should be determined. This account is yet determined in the purchase requisition and copied to the purchase order.

I've been searching for a while and in my view, it has something to do with the account assignment. I tried the transaction OBYC, but I don't think I can change the account there.

Could someone guide me through this problem?

Kind regards,

Samuel