Originally Posted by mozilla

This seems correct, but technically, you don't get the "option" of forfeiting. UA has already decided that you will be refunded and that you won't get anything back to the original FOP - nor will you be charged anything - when the residual is $300 or less.

This is a "Pacific Round Trip Refundable with a Fee Economy Fare." CAT31 specifies that there will be no reissue charge for voluntary changes to the 1st ticketed flight coupon before departure. When the change results in a higher fare, the fare difference shall be collected. When the change results in a lower fare, the residual needs to be subtracted from the change penalty and then add-collected or refunded. CAT16 generally specifies that changes are permitted free of charge; when it comes to changes to the 1st ticketed flight coupon, it refers to the cancellation section regarding refunds. The cancellation section specifies a 300 USD penalty for refunds and to refund the residual, if any, to the original FOP.

So coming back to CAT31, the change is completed free of charge. When the change results in a lower fare, the residual is subtracted from the change penalty of 0 USD. This will always be negative, so add-collect is irrelevant, and the residual now becomes a refundable amount. The refund stage consists of charging the 300 USD cancellation penalty to the refundable amount; any residual that is then remaining will be refunded to the original FOP (no add-collect requirement here).