Need a PO for unknown quantity with unknown value

Dear SCN User's,

I have a scenario like we raise a PO to vendor for 1 quantity 10 rs value but we dont know how much quantity we receive at what price he might send, I thought of doing value contract but I dont know the exact value also, so please help me out if there is any such type of PO where we dont know the exact order quantity but with a value for each quantity is know.

Thanks,

Rocky