Dear Amr,
If a credit management is activated against the customer and if an LC has been submitted against a Sale Document for which FD has been created via VX11N. This FD will then be linked to the Sales document Header and Item level and Credit Check will happen in case the order value will be exceeded by FD value which is standard behavior. In OVA8 you need to assign a routine 901 for Orders and 902 for Deliveries here take a help of OSS Note 303409 - Credit management and documentary payments. Test the same and i'm sure it will work fine against your desired query.