Dina D’malchusa is a specific Halacha applicable to Yidden. And choshen mishpat takes precedence to Dina D’malchusa regarding Yidden, in any event.
One of the Sheva Mitzvos is for goyim to establish laws and courts. That’s a more reasonable claim that secular law is binding upon goyim. If you go that route (which can be disputed since the other party to the transaction are Yidden, so really Halacha takes precedence over secular law) you’ll need to deal with probate court and estate distribution under state law, which will likely delay buying back the chometz for a several months or more.