While writing the question "So, how did the money exchange hands?" I came up with a hypothesis: I guess the Russian regime have offshore accounts and would've transferred the amount to another foreign account they assume to belong to the killer, but actually is ultimately controlled by the Ukrainian authorities.

If the money stayed in the Russian banking system, I can imagine they could've seized the recipient account, and transferring such a large amount out of the country would be difficult (although it's peanuts in the world of dark finances)