Form and program are assigned in transaction NACE (or just go into the IMG transaction OLME and there to messages) This is usually done by the application consultant.
But to be honest, a change form that just prints that the quantity was changed without telling the new quantity, is nonsense and cannot be send to the vendor except you want become a laughing stock at your vendor.
So you would not even need a test as common sense is already telling you that this can only be caused by a faulty own development