Credit for unused upgrade

A corporate vendor booked me an economy ticket to attend a training meeting in FL. I paid separately to upgrade to Comfort+. Unfortunately needed to cancel my trip due to a medical issue. I called the sponsoring corporation travel agent & they told me to go to Delta. As the wait was 45min I used messaging app. Live agent told me to have TA issue refund. This transaction was a separate purchase for $370 on my own card. Just looking for eCredit as I’m aware paid upgrades are not refundable. Any insights to get this would be really helpful. TIA