Ext. Delivery can't be saved

Hi,

When a returns delivery is created (ZLR), by referring to a returns order (YQRE). The PO number from the order is copied into the u2018external deliveryu2019 field of the delivery header (administration tab)

Currently this field cannot be changed in the delivery using VL02N transaction. When the PO number is empty in the order, this field is also empty. We get the following message

a valid PO number is required in Ext. Delivery field.

Even when a value is entered in the u2018ext. deliveryu2019 field, upon saving, it fetches the value from the reference order PO number.

The changes done to this field using VL02N transaction should be allowed to be saved.

can someone please tell me why it's happenning?

regards,

eric gao