In Purchase/Receive Inventory you can click the tab Apply to Purchases and enter the variance amount to your variance GL account.

Why are you trying to do this?   Just for your information?  The PO does not hit the GL so nothing has been recorded until you enter the invoice.  And if you over sold the item, Sage will do a system cost adjustment behind the scenes if the costing is not correct on goods already sold. 

I am concerned that you will not have the correct costing on items sold going forward if you try to make adjustments when purchasing.

We currently don't use any inventory items with costing because of our fairly unique industry but still need to record some form of accrued costs in period an invoice was issued (we basically  resell  services that we may not get billed for over a month and each of those services can have over five different vendors to choose from, each with different costs). So we basically utilize the PO accrual in order to get as close to the matching principal as possible