Purchase Order - Invoice Mismatch

Good morning everyone.
I am introducing the ERPNext system into our company, and we have just started entering the first data, this is my first message on the forum.
I have a problem with purchase orders and invoices.
In my order, I have an item with the list price included. The invoice received has a 20% discount on that item. I then entered the discount on the invoice in the “discount” field and the invoice total was correct. However, the total is obviously different from that of the order, so the order is “partially billed”
I hope the description is clear enough.
Thanks to those who reply