Imagine holding billions of dollars in frozen assets, but not knowing what will happen if they’re taken away. That’s exactly the challenge facing Euroclear, a Belgium-based financial giant that’s sitting on nearly €197 billion ($213 billion) of Russian assets frozen under EU sanctions. These assets, belonging to Russia’s central bank, have sparked intense debate across Europe about whether they should be confiscated to help Ukraine – but Euroclear is sounding the alarm.

Here’s where it g...