Hello ,
i believe , vendor will send invoice as per agreed pay terms , like 30 % in 10 days remaining post 60 days. And data entry person will enter what ever invoice is .
So if you control it with pay term vendor should send invoice of agreed amount and quantity only.and the user need to enter the same.
other approach will be split the Quantity in ,multiple line items in PO then do GR with the delivery note and then Park an invoice with delivery note. and let the accounting person to post it as per business requirement .
Thanks
kUnal