An episode that could fit the criteria is Star Trek: Voyager S5 E04 - In The Flesh.
Captain Janeway has a tense negation with Species 8472.
However, it's not the shields she lowers as a sign of good faith, but rather, disarming the ships nano-probe weapons.
JANEWAY: One of us has to take our finger off the trigger. It might as
well be me.
JANEWAY: Our defences are down. What will you do now? Blow Voyager to
high heavens, or do we keep talking?
ARCHER: Their weapons are offline.
BULLOCK: It's a trick.
The "One of us has to take our finger off the trigger. It might as well be me." quote could be the phrase you're looking for.
Full transcript: http://www.chakoteya.net/Voyager/504.htm