...little wrong with Kartik's suggestion; I could pull up some historical documents in German where it would say "additional pay," as they do here:
"In addition, all soldier clerks at the War Office receive 6d. a day additional pay."
https://hansard.parliament.uk/Commons/1940-04-04/debates/836...But I think it's unnecessary; there are many ways to say this, though I don think that either incentive or premium makes sense in this context (he no longer works there so there isn't even a base salary or wage to speak of).
I'm having more trouble with "let go," which is not only a euphemism, but I'd say (in this meaning) way more recent than the 1940s. Dismiss or similar would work better here, I believe:
https://www.etymonline.com/word/dismiss
peer agreement (net): +4