You don't understand how it works
They don't get bribes while they have their administration posting. That is clearly illegal and would lead to guaranteed jail time if exposed so only a few clueless idiots do that. Instead, after they leave they get big money as "consultants" that don't actually do anything, or for serving on boards that hardly ever meet. IF they play ball. If they don't, they won't get such offers.
No one needs to make any promises, no money needs to change hands in exchange for favors, so nothing is done which violates any laws. It is all just understood, because everyone does this and always has, in both republican and democrat administrations. This is the swamp Trump promised to drain, which has instead been getting deeper since he took office. Don't look for that to change, even if a "do-gooder" like Sanders is elected in 2020.