There are any number of threads on FT about exactly this situation. The answer is always the same and the reasons people are ask are the same (because they know the answer).
1. The value of the credit belongs to one's (former) employer, not the traveler. The fact that one is able to use the credit does not have anything to do with ownership.
2. Many (most) corporate contracts permit the employer to use credits for other employees. That is a relatively unique benefit for corporate customers. Thus, the suggestion that the credit will go to waste is generally false.
3, If the funds were paid by the (former) employer and used by the employee for non-business travel for that employer, the employee must report the value of the credit (when used) as income.
4. Before using the (former) employer's property, one should have a written confirmation that the employer approves this. Anything short of affirmative approval is somewhere between theoretical and actual theft / wire fraud.
5. The (former) employer may be required to issue a 1099 to the (former) employee for the value. This is a manual & time-consuming process and is generally the real show-stopper.
Rinse, wash, rinse wash. The answers don't change.