Changing the PO manually is not the right solution here. There are more issues related to it.

You receive in inventory at a price, and you get afterwards the bill at another price. The value of inventory must be changed, but there should also be a correction journal entry to get the right values in accounting.

I made a quick fix for it with python code on account_move (to change the value in stock_valuation_layer) and on stock_valuation_layer (to make the correction journal entry), but this is not enough. There are more problems with it when you return the 2nd receipt when the first is still in inventory.

I made a ticket at Odoo for it.

I can't adapt the PO! I don't have the information, yet. This comes when the supplier sends the invoice. 

Rather then doing it manually I'd expect the system to correct it afterwards automatically at the moment when the purchase invoice arrives. The problem in the current behavior is that the inventory value doesn't equals the P&L postings. Easy to check that for an auditor and that is sth you don't want to happen in your company, right? 

not perfect world and solution some has to be done by hand :)
so you have PO 3 qty. price 10eur.

when you receive 1 and bill, and waiting for others. you need to correct PO.

so i correct PO. one line -1 unit 10 eur(which received already). and other line 2 units, for 20eur. and when i validate receipt. validation ok. because transfer get value from PO.