When an item is added to your purchase order, it can only be an item that already exists in the job budget.

If you only partially receive items against a PO, the invoice can be partially applied, and the remaining amounts will be displayed on the right-hand side of Add/Edit PO screen.

If the remaining number is green, you still have money left to spend, and red means more has been invoiced/paid in relation to that item when compared to the amount that was originally budgeted.