Unfortunately, they can't:
Also also, if China were to create a new mission to collect them, they would see their costs balloon to unsustainable levels as well, as the issue is how the mission is designed. The samples are all over the place and would need a rover specifically designed to collect them and return them to a launcher.
The US Mars sample return project was always half baked in the hopes that doing something would garner funding for the rest of the mission.
That alone is a separate multibillion dollar program just to do that.
Sounds like they blew it. What a shame.
https://www.nytimes.com/interactive/2014/03/23/world/asia/23...
https://www.nytimes.com/2014/03/23/world/asia/nsa-breached-c...
> A State Party to the Treaty on whose registry an object launched into outer space is carried shall retain jurisdiction and control over such object, and over any personnel thereof, while in outer space or on a celestial body. Ownership of objects launched into outer space, including objects landed or constructed on a celestial body, and of their component parts, is not affected by their presence in outer space or on a celestial body or by their return to the Earth. Such objects or component parts found beyond the limits of the State Party to the Treaty on whose registry they are carried shall be returned to that State Party, which shall, upon request, furnish identifying data prior to their return.
It is clear, then, that the sample containers are still the property of the US, and that if the Chinese brought them back to Earth, they would be required to give them to the US.
[1] https://www.unoosa.org/unoosa/en/ourwork/spacelaw/treaties/o...