You don't need the trusted third party. You could share the hash of the numbers so they couldn't be changed during the process, then do the millionaires problem [1] to find out which number was bigger, and finally and only if the right one is bigger, share the figures (setting the salary at the midpoint is the same as sharing the figures in terms of information transfer) and calculate it that way. The only way to not reveal the offers after a successful round would be to introduce some randomness, maybe choose a number at random from some agreed portion of the range between the numbers. I don't know if there is a zero knowledge protocol for selecting a number at random from a range where the extremes are set by two different people.

[1] https://en.wikipedia.org/wiki/Yao%27s_Millionaires%27_Proble...