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There is no way to create a delivery note against a purchase order. Please use the feedback tool under the cog wheel to suggest this feature to the developers. They will then look into implementing this in future updates if possible.
Sorry I think you have misunderstood. I do not want to create a delivery note against a purchase order. I want to receive the delivery against the purchase order, as you could do on QuickBooks Pro.
So for instance, if I have ordered 50 terminals but only 30 have been delivered, I want to be able to see that I'm still waiting on the other 20.
When you create the PO, select copy to bill and amend the quantity to the amount you actually received, 30 and then save. When you refer back to the PO it will still show the full quantity you ordered however it will show a linked transaction, the bill. If you then receive the remainder of the PO, select copy to bill and the bill will then populate with the remaining quantity, 20.