FWIW, I’m 99.99% sure DL would treat the equivalent situation in a substantially similar way. The reason is as Lux Flyer alluded — international one-way flights are often substantially more than half of the cost of a round-trip ticket. The airlines do this because business travelers are much more likely than leisure travelers to buy one-way international tickets, so it’s a way for them to achieve price discrimination. If any airline were to let you have half the cost of your ticket back as FFC in order to take any other flight in their system, it would essentially submarine their own revenue protection — nobody would pay the one-way fares.

Which is the thing to watch out for — you say that you can find a reason to return to Europe — but keep in mind that you’ll need a one-way ticket back, or you’ll need to have a round-trip that starts in Europe to get you home, and then leads to kicking the can down the road. Make sure you price the return out before making your plans for the unused ticket credit. (Award tickets are often a better deal than paid tickets for one-way international travel).