How to deal with down payments in purchase order

Summary:

How we can handle downpayments in purchase orders?

if we have a device for example (Quantity equals to one) and we have a downpaymet about 20% from the total price , how we can handle this case since the reciept will be for the whole quantity which is one in our case , so the total amount thats open to invoice will not be corret.

Split the lines will lead us to change the unit price which is not true

any help?

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