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This is Standard Functionality.
When you are entering amount based document lines in purchasing windows such as the Purchase Orders window,
Purchasing skips the Item and Revision fields, and sets the price to 1. You simply provide the total amount
of 5,000 as the quantity. Then, when you record your receipts, you can enter the amount you have received
as a quantity. For example, if you have incurred $300 of a $500 charge, you record a quantity of 300.
You can refer below note for the same,
Amount Based Line Type Will Not Let Choose Item (Doc ID 230079.1)
Thank you for the feedback and confirming the behavior as standard functionality.