What options are available for extending a trip credit?

I have a flight credit that looks like it was converted into a trip credit at some point that expires towards the end of August. Based on what I had read elsewhere, I thought I could book a flight with a lesser value than the credit and that would extend the remaining balance - which doesn't seem to be the case after booking a flight yesterday.

No upcoming trips planned - all I have right now is a trip credit balance I'd like to extend beyond the Aug 2024 expiry and a flight in a couple of weeks that I can't take and is within the 24 hr cooling window.