expression
that's a different matter; that's another story
Used to separate a new condition from a previous statement, often after a conditional like 〜となると or 〜なら. Indicates that under the new condition, the situation changes.
普段は安い店で買うけど、贈り物となると話は別だ。
I usually buy at cheap stores, but when it comes to gifts, that's a different matter.
彼は普段はおとなしいが、お酒が入ると話は別だ。
He's usually quiet, but once he's had a drink, that's another story.