I usually only dress formally when I’m presenting outside of my company and people usually joke about my having interviewed somewhere else that day.
A person in my life changed jobs to a worse location and then changed back. Pay rise of like 20% and a better position (senior doctor). That is just not justifiable. Same employer, same hospital, same ward.
It's fair to argue whether it is moral to pay people less than what you think they are worth to you. That is questioning the morality of capitalism.
In contrast, "fighting fire with fire" is justifying lying. Maybe the ends justify the means in specific cases, but I believe that generally people and particularly ethicists agree that lying for personal gain is generally considered immoral/unethical. Of course, there are cases where lying is considered justified and the moral thing to do e.g. saying no if a Nazi asks you if you're hiding Jews in your attic. But in that case, you're lying not so much for personal gain but to help others.
For example, your manager would like to give you a raise, but company policy or unwritten "this is how it works" rules say that a raise of more than 15% can only be given when the employee has received an offer from another company. But they don't actually want to see the offer, they just want you to say it, just as your partner wants you to say "I love you" to him or her from time to time to continue receiving the benefits you have been looking forward to maintaining or increasing.
Withholding the truth is also lying, by omission. In that sense, your employer is already lying to you by not compensating you according to your actual value. I realize it’s a bit of a stretch but there’s something there.
Lying to get a financial gain is the definition of fraud.
That said, it's one of those technically illegal things that no one in a million years is going to get charged for.
However, I wonder if it actually is fraud. Intent matters. The intention isn’t to defraud the employer, but rather to short-circuit the whole process of job skipping for a correct wage. So it’s actually not an unfair gain, and that is precisely why I think it’s not even immoral in this case.