There is far more to U.S. government than the Constitution, which is only a framework. There are an enormous body of law, court precedents, institutional customs, federalized government, and of course public opinion.

Among that body of law are existing tax rates, which are currently over 15% for corporations. Under the Constitution, the President must agree to changes in tax rates unless their veto is overridden.

You're right that Congress theoretically could violate the agreement, but in practice, it's almost irrelevant. They could theoretically pass a bill tomorrow that eliminates every tax and every law in the U.S.