D&D 5e does a fiend drop any items carried when it is killed and returned to Avernus?

I had a bearded devil carrying some keys that would open a jail cell, tied to his belt and was slain. My question is, does it drop the keys and can be retrieved by my players or would they go with it back to Avernus?

I am not too concerned if I have to make my own ruling but if this has been answered somewhere I would prefer to use that rule.

Many Thanks.