There was the assets belonging to Iran after the US embassy in Tehran was invaded. Libyan assets I think were frozen during the Reagan years. And I suppose Saddam’s assets during Desert Shield. Have they been returned?
They were unfrozen as part of the deal by which the hostages were released in 1981. Part were immediately returned to the Iranian government, part were used to settle Iranian debts to US and European banks, and part was tied up in disputes about who owed what to who. The Iran-US Claims Tribunal was set up to resolve those disputes and determine who gets what. That work is mostly but still not completely finished.
In civil law here in the US, we have the ability to implement FIRE (Frozen, Indicted, Restricted, Encumbered). Generally, that stuff isn’t going back to it’s owner - it’s a mechanism for preventing criminals and such from using their assets to continue causing harm.
On the international scene (where I’m no expert), we have the Office of Foreign Assets Control and ITAR to give us leverage. Certain things cannot leave the country, which means money at times.