Baba Kama 58a - Dina D'malchusa

In R' Asher Weiss's weekly shiur on parshas mishpatim he quotes 2 fundamental approaches for the source of dina d'malchusa. The Ran and Rashbah take the approach that the land is owned by the king so he can implement what he wants. Whereas the Rashbam (baba basra 54b) and the Rambam (Hilchos gezeila 5:18) take the approach of "heskem b'nei hamedina", meaning that the inhabitants of a particular area accept certain rules and principles on themselves (there are also more approaches such as rashi in gittin who implies that it is based on the fact that goyim are commanded on dinim, which gives them authority to impose their dinim even on the jews). Based on the approach of "heskem b'nei hamedina", the rambam makes a distinction between an actual king and a warlord or mafia, since those groups may be powerful but have not been accepted by the population. This approach also seems to be the basis of the Ramah quoted in Tur (c.m. 369) who says that tax increases that are being imposed against popular opinion would not be justified using the rule of dina d'malchusa.

Tosafos about mavriach ari, at the very end speaks about Jews who were forced to escape from their land to avoid taxes that they could not afford. The land would then be taken by the government and sold off to others. The R"I holds that if the government sold it to another jew, the sale is not binding since they had no right to sell it in the first place (but the original owner would have to compensate for benefits he received). Tosafos explains that this is not dina d'malchusa, rather it is gezeila. Tosafos doesn't consider it gezeila due to the lack of authority such as a warlord or mafia type, rather tosafos says - ואם יש שר שבא לשנות את הדין ולעשות דין לעצמו אין זה דינא דמלכותא שהרי זה הדין אין הגון כלל

Tosafos seems to understand that the concept of dina d'malchuas is limited to what people consider justice, not what people perceive as theft. Therefore, even a government who technically has the power to impose dina d'malchusa, they can only do what is considered by most people to be "fair". This would also be based on the approach of the Rambam and Rashbam that the concept of dina d'malchusa is based on consensus of it's citizens.