It's from Where No Duck Has Gone Before. After Scrooge gets the phone call, he says something along the lines of "But they can't repay me in cash!" If you watch that bit of speech in slow-mo, you can find the exact frame.
http://coucoucircus.org/da/index.php
This site is a goldmine for French TV songs, though if you look hard enough, you can find English versions. Scroll down to I to find the Inspector Gadget songs.
From what I can tell, this song was exclusively French. It's really a song about Finot (Brain). God only knows what the song was used for. The real French theme is here. http://inspectorgadgetfrench.ytmnd.com/
