Suggestion:
User can take/create new item series for promotion item .
User can simply do the Purchase flow, Purchase order to receipt as done in a normal item.
If the price list is found for the supplier and price list contains a price for the item, the PO retrieves the price from that price list. When it is directly ordered with or without any reference/source, the System allows the user to enter the rate to be applied at the time of receipt.