Why North Korea is sending soldiers to the Russian front lines | Sending solders to help Russia’s war effort against Ukraine could earn valuable foreign currency for Kim Jong Un’s regime and bolster their strengthening ties.

Which “valuable foreign currency” is that? Russia has exhausted most of its actually “valuable” currency. About the only thing it has in any volume is Chinese Yuan, and they need as much of that as possible to trade with China. If Kim thinks RUB are more useful than KPW then he is even stupider than I thought.
I doubt currency is really the important thing. Likely they are being paid in grain since Russia has that in volume and avoiding famine is probably the single most important regime safety task in NK. A secondary value is getting KPA soldiers real combat experience, because Kim is delusional enough to think he can take on South Korea if only his troops have a little experience. I suspect ROK soldiers will surprise him, even the soft K-Pop boys. And quality of equipment and training will somewhat offset lack of experience and grit on the ROK side.