This article is for case when you have created a purchase order and need to prepay for purchase orders. First you would need to go to Admin, Enterprise Setup, Entity settings, Check the checkbox
Show Purchase Order Item Prepayment inside Bill Item Form section. Save the setting by clicking on Update Configuration.
First create the purchase order. To pay for the entire purchase order or a percentage of it, create a new bill and under the bill item form, Click on the list under Prepayment for Products on P/O. This shows you all open PO items without prepayment, choose the items you are prepaying. A new bill item is then created. The quantity on the bill item is locked to the total quantity on the purchase order line. To change the amount you are paying for the PO, change the price to control the final amount.