Rtv to purchasing/budget

HI users,

I want to do a Rtv. First I did a purchase order with an item that had 3 as quantity, approved, budget went through and dispatched, next I did a receipt with the same quantity as the purchase order. So far so good. I close the PO and went on to do the rtv. I did the RTV but I only wanted to return 1 of that item with an action of EXCHANGE. Approved and dispatched, but the budget wasn't updated. 

My question is how can I update the budget? because if you return 1 that would surely mean you can get your money back, reviewing the budget it shows that it returned the same amount as it first went through instead of the amount of that only item I wanted to return and how can I do a new PO thus updating the budget?