Qitzur Shulchan Arukh – 65:30
A [Jewish] apostate — it is prohibited to borrow money from him on interest. And also lending to him on interest, it’s also appropriate to be stringent.
We close the siman on ribbis by getting back to the point. Is the verse’s “achikha” inclusive of every Jew, or only of Jews who believe in Judaism, shomerei Shabbos?
Not that this issue comes up much, since a mumar is someone who rejected a Torah upbringing, not someone who simply wasn’t exposed to the traditional Jewish worldview, or his exposure was in a context where they were prejudiced to reject it. (“See son, this is what those Orthodox people believe, but we know better…”)
Starting Monday, be”H, we will be moving on to iska, the rules of doing business — including some very-close-to-ribis looking deals.