The casus belli for an Russian invasion of the EU is going to be this:
Give the billions in money to Russia or else:
Seizing foreign assets in this manner raises a number of serious legal and policy questions and may have consequences for the broader international system.
www.brookings.edu
If the assets are frozen with the ability to return to Russia, then Europe is safer from a Russian invasion.
If the assets are donated to build Ukraine back from the unethical war on Ukraine, Russia would use that excuse as casus belli to do exactly the thing it wants, more war, more excuses, more annexation. Next - EU territory. Moscow would say the reason why EU is being invaded is because EU stole their money. When Russia stole Crimea and waged a 8 year hot and warm war with Ukraine, before deciding to steal more land. Moscow manufactures the excuses, then invades and makes itself appear the "victim" to the dupes and morons.