Hi,
Unfortunately in sap there is no process of variable prices at the PO stage . However this could be mapped like this .
Create the material Onion with V MAP .
Create the Contract for 700 KG at an average price . ( no problem . you can change the PO price . Does scales exists ?
Create a PO every day based upon contract on that day price. for sap the PO price should be known even during creation of PO itself as it is time idependenat condition . and you can post the MIRO for that amount billed and the differences will be treated accordingly for the MAP and posted price and the current price for material changes . and proceed so on .
Other PO process like invoicing plans / blanket PO will not work .
Regards,