Hi Irfan AB,
1) First of all he can go to PO History Tab in PO and check for GR and IR documents.
2) While doing IR generally we provide the quantity (if GR based IV is not active) and system picks up the Amount by multiplying the quantity with the price stated in the PO. So, as you are saying he has done IR for actual (total) amount - that means he has booked total quantity of PO in IR.
3) It infers from the above statement that - he has mentioned Higher Quantity (than he received i.e. zero) in the IR - That is nothing but Quantity Variance.
4) He can create Credit Memo on the Vendor - by using PO Reference - for the quantity / amount he did IR.
5) When Vendor sends the Goods actually, then GR can be done as usual for the quantity stated in PO since the present GR is for zero quantity
6) And do IR accordingly.
Hope it helps.
Regards,