Legally, who do the credits belong to? Even if the credits legally belong to OP, does the employer have a valid legal claim (restitution? unjust enrichment?) against OP for $1500 (or some lesser amount representing the FMV of a $1500 travel credit)?

From reading this thread, seems like the credits legally belong to OP if they're in his name. But you could make the case that the employer might have some sort of a plausible legal claim against OP for reimbursement. Very small chance they'd ever do anything about it given the small amount involved.

Ethically, IMHO, OP is totally in the right to use them and tell the employer to buzz off if they seek reimbursement.