Buying Tiles is the original and sure way to get REKK.
The rule is simple: each time a Tile (or more usually, Tiles) is available on the Map and is bought in dollars for the first time, you get the Tile(s), and you earn 1 REKK at the same time. That’s it.
When somebody else buy your Property and this Tile, the REKK stay yours.
The Tile and the REKK are created in the game at the same time but from there, their fates diverge. Sell the property, keep the REKK. Or keep both. Or keep the Tiles and sell the REKK.