I think ultimately you'll get a defect anyway when you give the money back, as it triggers automatically in instances where a full refund is issued. I think the only way to issue a full refund but avoid a defect is to issue the amount less one cent, then send a separate transaction for the final penny. I may be misinformed though.
That may be true about an automatic defect. But, end of day, I'm not sure that it matters much, particularly if you're dealing with someone unwilling to resolve something informally.