I am not disagreeing with Judy but I would like to add that you can use "fall / drop in salaries" in a more formal context, probably more when writing. You could say : "There has been a sharp drop / fall in salaries in Europe over the last year". You might see this in a newspaper. I agree that it has to be "in" not "of"

Yes, fall (in) and drop (in) can both be used for talking about decreases.

I would not use either of them. I would say . I would just say his salary has gone down instead of up. Or the salaries for teachers are being cut each year.