The other day, I learned "a the phrase "deal breaker" which means a critical disadvantage on negotiation.

"Deal breaker" is actually not a disadvantage. It's a condition/issue/attribute that could cause a person to withdraw from a negotiation. Disadvantage assumes a negative attribute, but a deal breaker is a subjective situation where the other party specifically dislikes something.

It's also used casually, outside of negotiations. For example, if a person doesn't want to date and says, "She snores when she sleeps. That's a deal breaker for me."

At first, I thought a deal breaker iwas an agent who underhands in order to breakedly ruined a deal down.

Not sure if "underhand" was the word you wanted.

Besides, this word reminded me of "Imagine breaker" which is the name of special ability infrom a light novel.