Follow the money. £500k teaser

I’ve been sent a brain teaser.  What legal reasons would someone have to discretely pay another individual £500k?  The people are not currently in business together, although the recipient previously worked for the sender.  No goods have been provided.

It’s a puzzle.

Looking for legal reasons only, no crazy illegal notions or wild speculation about real persons, this is purely a theoretical exercise.

Apparently I’m being sent a moral dilemma tomorrow: Will the recipient, with off-shore accounts but living in interesting times, declare the £500k?