Let's not delve into mitigating factors such as unfair practices or undue influence for a moment. A contract is a promise. After signing it, I am bound to the terms. If I am paid more for my efforts, that is ex gratia, the way bonuses work. Nobody forced the bus drivers to take on the job as it was obviously an attractive proposition for them vis-a-vis other opportunities in the PRC.
The guiding principle is how much your word is your bond. You have to protect that reputation. Had SMRT not been in the wrong by reneging on their original contract terms, I would say the PRC drivers deserve what they are getting.
We now know (at least I just remembered) that SMRT had renege on terms with all unionized bus drivers, and as I understand it that is primarily because union representatives can make agreements with SMRT management without union member consent or quorum. The renege led to a long drawn faceoff, the result is a two day strike by 184 PRC bus drivers. It should not have been able to change contract terms at whim. In this context I tend to side with the bus drivers on their actions.
So, your work is not worth the few hundred dollars more? If this is what you think, I would rather employ the angmo and pay him more for the work.
You want to know why?
Before the contract is signed one could still argue, but not after the fact. There is always the option of leaving for greener pastures, as I have done several times myself.