People are free to try offer better deals and services though often times they'll likely be much worse.
Offering better deals in this area almost always means offering horrible services.. I don't think it is fine to do this, especially not when the user was already hired. If you allow this, then you essentially allow users to do this out of spite; slippery slope here and there, you reject someone's entry in a bounty because of them being a farmed account: They attempt to get you ("legitimately; by offering "better" services") fired.
It's a shitty thing to do but not illegal. The only issue I would have is if you had a deal with a company and they got rid of you during the campaign just to save a bit of money. That would be a breach of contract.