How to set the Purchase Receipt Rate after the Item is Sold (i.e Sales order is completed)

Hi there,
Hope you guys doing great.

I’m having a scenario which I’m unable to solve through EPRNext, or I guess I’m missing something.

Here’s the Usecase.

I created a Item in ERPNext, that particular item is given to me by the supplier without any pricing in mind.
Now suppose I want to increase the quantity of that product without assigning a COST to it. How can I do it?

What my use-case is,
I want to first sell the product (offline) and then DEPENDING on that price taking a 20% cut I want to generate a purchase receipt against the Supplier.

My GOAL is that we don’t know at how much will the product be sold, SO only after selling I will get a Idea of what I have to bill to the Supplier.

I hope anyone can provide me a way out of this or at least some kind of alternative methods to achieve this.