Suggestion:
User can get the manufacturer code against the receipt line item added on PO selection. If user maps the item to its manufacturer in Item Manufacturer Master, then after the item is selected in Receipt the manufacturer code will get populated automatically.
Note : If approved supplier in site item/item master is N then header supplier code will be set, else system checks item manufacturer for the item_code if one record found, then system sets the same supplier code manufacturer else it is set blank.
For Item Manufacturer master goto SCM->SCM settings-> Masters-> Item-> Item Manufacturer