Csdrhrt

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.

The most simple answer (but it can be unsuitable):

A and B can use public-key cryptography (e.g. with Diffie-Hellman key exchange). The problem becomes even easier because the message (the salary) to send is a number. So, all the messages being whispered will be in the form "Tell A(B) the number 50694, please".

B starts the conversation with any number B. When it gets to A, A says SA+B, where SA is A's salary. When this gets back to B, B now knows A's salary (and no-one other than A does).

This can be :)

B sits on the left of A and ask A's salary. And no one know what
B ask to A. Then A whispers his own salary to the person on the right.
Finally B get's answer! :)

B sits on the left of A. A asks for B's salary and B whispers his/her salary to A. Then A whispers his own salary+B's salary counter-clockwise. When B hears what A whispered to others, A tells him/her to substract his/her own salary from it.

And now for something completely different:

A and B both speak a language
(for sake of completeness, let’s say Vulcan)
that none of the others know. 
But the others can repeat Vulcan phonetically. 
So A whispers their salary in Vulcan to the person to his right,
who relays it (phonetically) to the next person, …. 
Eventually it reaches B, who is the only person who can understand it.

I will speculate here a bit. Given that everybody passes exactly what he/she got and does not change anything.

If A knows B's salary, then he can just pass something like it is 700 more than yours

If A and B could communicate before the game started, then they could figure out some function to determine it and pass the numbers. For example: $F(x,y) = x^y + y*x$. So saying 14 and 3 would mean $14^3 + 14*3 = 2744 + 42 = 2786$

They could also use some coder/decoder to pass it if they both know the algorithm that's being used

Try this

B tell his salary to one who sits beside him, and when A receive that, A add that number with his salary, and pass it to one who sits next to him and add a message saying that B have to subtract with his salary. Say B salary is 20K and A was 30K then A tells the person next to him as 50K minus B salary.