Purchase an item in a UOM other than Primary UOM, when the conversion rate varies on each purchase

Dear Gurus,

We are replacing SAP with Oracle and stuck with below requirement from users.

In SAP there is functionality where buyer is able to give a conversion rate between UOMs manually even for a PO without Item. In the below example they want to order 8 PCS of PIPE and pipe weight in kG is vary from supplier to supplier .

Example :

Item : Pipe
Primary UOM : Each
Secondory UOM : KG

1. Purchase Order issued to "X" supplier where Each Pipe weight in KG is 128 KG.
    Item : Pipe
    Primary UOM : Each
    PO Quantity : 8 EACh
    PO UOM conversation rate : 1 EACH =128 KG

    Invoice will be sent for 1024 KG (8 * 128)