International Emergency Economic Powers Act - Wikipedia Trump's trump card. An authoritative move. Freezing the assets and transactions dealing with Chinese companies/nationals. China will have a very hard time obtaining U.S. currency. Interesting times we live in.
The reason the Chinese didn't devalue their currency further is that when interest rates were essentially 0% a significant amount of development in China was funded with loans from US banks in dollars. If the yuan devalues too much the developers holding the loans would go bankrupt trying to pay back the loans. The
If the Chinese couldn't get US currency, they can't pay back the loans it could badly hurt the US banks which could trigger problems throughout the US economy.