A relative equality of power is not required for there to be true consent. True consent simply requires each side to understand the deal sufficiently to genuinely believe that they are better off taking the deal than rejecting it.
But surely you wouldn't argue that someone handing over their wallet in exchange for not being shot was consensual, even if the person really truly preferred being a little poorer to being dead. So presumably your opinion would be that they did not have the third option, which is to say "no, i'd rather not give you my money /or/ be shot" and walk away.
However, since we cannot walk away from the table of "making a living" completely -- (those born landless are forced to sell our labor to the owning class to literally survive) there must be mechanisms for making that relationship as equal as possible.