To fill in a few blanks:

The coupon has no identifying code, and the issuer has no way to track it even to my account, since it was not originally issued to me in the first place.

I'm pretty sure, based on the specifics of the situation, that it was not comped. I've had that, happen, too, with Delta PMUs where I qualify for the upgrade on status alone, and so they don't even ask for the PMU certificate. In those cases, I have no problem keeping the PMU and using it again (or trying to... they're actually pretty difficult to use for domestic flights). But this situation was different.

And my thought was not to trade the coupon for any personal gain. I initially received the coupon as an outright gift via FT's Coupon Connection, and my thought was to offer it to someone else, for free. But I think I am sensing that at least some people also see the potential ethical problem that bothers me a bit...