Although many people have given this same general idea, it seems like they have all over-complicated a very simple solution. Assuming the other members of the circle are cooperative, and pass along messages accurately:
All B needs to do is pick any random number and pass it to the right. When A receives it, he subtracts it from his own salary and passes along the new number. When that reaches B he can reconstruct the original. Only A & B know both numbers. People between A and B (to the right of A) know the encoded salary only, people between B and A (to the left of A) know only the code number.
The instructions can be passed quite openly around the circle assuming the other members are cooperative, honest and accurate.