be another matter

To be a separate and very different situation, topic, idea, etc., that has a whole other set of difficulties or problems to consider or contend with. It's all well and good wanting to help your fellow man, but inviting strangers off the street to come live with us is another matter entirely! Oh gosh, the state of the roof is another matter—we don't have time to get into all that right now. Getting a bad grade is one thing. Hiding it and lying about it is another matter entirely and totally unacceptable.

be another/a different ˈmatter

be very different: I know which area they live in, but whether I can find their house is a different matter altogether.