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

Scenario:

Co wants to buy 10 Feet of Pipe  (Feet is primary UOM: receiving, usage is done in Feet)

Vendors charge the price of pipe in Kgs.

Sometimes 10 Feet will equal 35 Kgs, on another occasion 10 Feet will be 32 Kgs.

Purchase Order needs to be made in Kgs (Mgmt. requirement)

Problem:

System calculates the purchase amount based on standard conversion every time:

Eg: 10 Feet of pipe is purchased. It weighs 35 Kgs. vendor charges 35 x $2 = $70

       PO Receipt of 10 Feet is entered.

       Inventory receipt value (Mtl Txn) is only $60   (Std Conv: 10 Feet = 30 Kgs)