When entering a receipt for a purchase order, the post amount box is calculated by multiplying the quantity received by the unit cost from the PO line item. When changing the unit cost on the Receipt the post amount does not update like the extended cost box. The Post amount calculates based on the original PO line item's unit cost.
Whether or not a PO is fully receipted depends on quantity received. The unit cost must remain consistent between the PO line items and the Receipts so that the encumbrance amount posted by the PO line item is fully reversed by the Receipts. Once a PO is fully receipted per the quantity received, the encumbrance amount must be fully relieved by the receipt posting. For this reason the unit cost on the receipt will always match the unit cost of the PO line item.