Incorrect refund balance showing

[vssc]

Allo. Need some assistance regarding an item exchange. Now, since Shopify doesn't have an exchange feature, I end up unfulfilling and editing exchange orders. But for some reason, the balance doesn't show up correctly. The image below shows a second exchange, and while the balance should be 0.00 (I have refunded the customer Rs 100 as the new order total was Rs 100 less), the order page says the customer owes me Rs 100? Why is this happening