Please point to the source, proving "Having the car in the US for 72 hours is the law" ? Maybe I did no read it all, but the link belowHaving the car in the US for 72 hours is the law unfortunately.
The first thing they ask you when you enter the export office is where you are coming from. If you were not in the US with the vehicle already then you would have to lie which I would not recommend doing to armed customs officers
I know there are people who have lied and exported right after crossing the border into the US but it is not a good idea.
https://www.cbp.gov/trade/basic-import-export/export-docs/motor-vehicle
states that only required documentation must be submitted at least 72 hours prior to export and the vehicle must be presented to customs at the time of exportation.
- Exportation at land border crossing points. For those vehicles exported by rail, highway, or under their own power.
The required documentation must be submitted to Customs at least 72 hours prior to export; and
- The vehicle must be presented to Customs at the time of exportation.
- Exportation at land border crossing points. For those vehicles exported by rail, highway, or under their own power.