Delta unable to apply oldest (expiring) credit to a fare first if the second credit covers the full fare?

Hi there,

I've been on the phone with Delta for the past hour and a half because I have a $~500 credit expiring at the end of the month. A few days ago, I booked a flight that's $~1750 and I called in to ask if they could apply the credit. They said sure, but they would have to cancel my ticket, apply the credit, and I would be left over with a ~500 credit expiring next year. Fine, I'm down with that.

90 minutes later, they've "tried everything" and can't figure out a way to do it and their only suggestion is that I keep my $~1750 credit, re-book the ticket with the $~500 credit and pay cash for the rest of the ticket which is the dumbest fucking thing I've ever heard.

Please, someone tell me there's a real solution here.