I would not be able to feel good about just using them. A very quick and very short email to the former manager, travel dept, and/or HR contact saying you have credits from a cancelled trip, what should you do with them and leave it at that for them to answer.
They've likely written them off already, or will not care, but agree that it's better to have them say it vs. just using them.
If they want them back, they should own working with AA to do so since it is their credit from their travel (and you won't be able to get cash from AA to reimburse the former employer). They should not be able to request any payment directly from you.