Booked with Cap One travel portal once to get a statement credit. Canceled the flight with DL. Then called DL to use the credit. Delta could see the credit with the ticket number, and Delta could take the ticket over for $50 (which they wouldn't waive).
Credit was for $400. I needed a flight for $300. C1 travel policy is you can use the credit once and forfeit any remaining balance, you can't hold it with the ticket number like you could with DL. It is in the T&C, though I had never seen that.
So I decided instead to have DL take over the ticket so I'd at least keep the $50 credit. But DL told me I had to pay the $50 up front to take over the ticket, I couldn't pay the fee out of the credit. So I ended up booking with C1 and letting them keep the remainder. Incredibly frustrating (and not Chase related) but worth considering in case Chase has a similar policy.