Frieght amount paid to lorry vendor

Hi Guru's

My requirement is like this:
PO - 20 tons
GR 1 - 10 tons + freight cost (lorry freight) Rs 1000
GR 2 - 10 tons + Freight cost (lorry freight) Rs 1500

We need to pay freight cost to lorry driver not to vendor. the final bill from vendor comes after 10 - 15 days. Since the freight cost is not fixed value I can't plan in my PO. So how to enter this freight costs before doing invoce verification for actual goods bill.