Do you have negative inventory turned on? If so, does this purchase bring your inventory quantity back to 0 on hand? If that is the case, then you must have 0 Value as well. Check what you have on hand now, then add to it what's on the PO. It sounds like it will come to 0 on hand. You will have to do an inventory adjustment to get a quantity and value into inventory, then record your PO then adjust inventory back.