Verb: He bargained with the city council to rent the
stadium.
No, bargain can not be an adverb.
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As a verb: I had to bargain with the shopkeeper.
As a noun: This thing that I bought was a bargain.
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This is not an idiom because it means just what it says. If
something is a bargain, then it is always a bargain. It doesn't
matter what the circumstances are.