What is the difference between the word "عام" (ʿām - year) and "سنة" (sanah - year) in His, the Almighty's, saying: "And he remained among them a thousand years save fifty ʿām (years)"?

There is no difference in meaning between the words "Aam" (عام - year) and "Sanah" (سنة - year). The preference for distinguishing a thousand with "Sanah" was for brevity, and for distinguishing fifty with "Aam" was to avoid repeating the word "Sanah."