Voiding Transaction ..clarity about + or - in Tender window

When we try to return a transaction and refund money, POS always causes us confusion because the dollar amounts in Tender screen don't appear negative. Why is it that it doesn show negative $100 or ($100.00) instead of $100 when it is actually a refund? It's a little confusing and I was hoping someone could suggest some rule of thumb for how we can be sure that the right amount appears on the screen and that we don't make a mistake of doubly charging a $100.00 instead of refunding ($100.00).