When people say only Congress can appropriate money or only Congress can change taxes, they don't understand that Congress DID do that. Years ago. They signed these decisions over to executive discretion...
For instance, President Trump can't unilaterally cancel people's payroll tax for a few months. UNLESS Congress ALREADY passed a bill that allows it. Which they did: 26 U.S.C. 7508A...
That law says (in a disaster) the Treas. Sec. can determine tax liability of those affected by the disaster for up to a year...
20 US Code 1085 allows the secretary to define an economic hardship and offer a deferment of federal student loans for up to 3 years...
So the question is not whether or not Congress passed laws. The question is whether the laws CONGRESS ALREADY PASSED have been legally and properly invoked.
The point is, the President is not legislating, he is invoking pre-existing legislation instead of relying on Congress to pass new legislation.