The other side of this coin is that russia siezed ~$50b from western companies and individuals before eu freezed their assets (actually that was a rationalization for doing that)
so, that's $250b left frozen.
Letting that much money just sitting (devalue) makes no sense, so collateral seems like a good choice...
The question is from whom did they borrow the money?
And the obvious answer to, who is going to pay this loan, is us... because we have 10t eur "just sitting around" that they will confiscate in some way.