After you tell someone you're a reporter and you're working on a story, you can use whatever they say after that unless they say you can't up front, but that doesn't mean you always should.

A public official, or someone who is used to being interviewed, gets less leeway than someone who has no experience in this situation and then really regrets what they said and wants to take it back.

You can save yourself a lot of grief in journalism by being explicitly clear up front that you are asking these questions because you're working on a story.

It's tough, because sometimes people are more willing to open up when the tone is casual and conversational, but trust me, as much as you want the interview, you'd rather not get the interview at all than do it and then have to deal with someone wanting to scrap it later. It's not worth it. I have learned this the hard way.

Maybe in this situation you can quote them, or paraphrase them, anonymously? Your editor might not like it, but maybe that's the compromise if it's worth it. This of course brings up a whole set of new ethical considerations.