Call DL. They may well charge a $50 ticket takeover fee but I'd consider that small potatoes for ensuring you don't lose a $10K credit.

As a data point, I recently booked a ticket through AmEx Travel on DL (to use an AmEx promo) which is powered by Expedia in the background. I later had to cancel the ticket. I canceled the ticket through DL.com and after canceling it, the next web page and subsequent said my ticket had no remaining value and I thought I had lost the ecredit and nothing was in my account. I called DL who said the ticket got hung in the system and the agent pushed it through properly and the ecredit was in my account. I was later able to use the ecredit on another ticket and the full value was applied to my purchase price. Long story short - just call DL and cancel through DL but I would recommend calling given the value of the ecredit.