The constitution does not actually forbid "abusing a position for financial gain", and thus it is left to the political process to address any such actions (voting for a different candidate), or the legislative process (defining certain acts as forbidden) – or, the impeachment process. The court system in the US does not have the power to decide on their own what politicians can and can't do, if there is no underlying law. It is within congressional power to define limits on the act of any politician, for example Congress could pass a law requiring the President and Vice-President to have no business interests or stocks during their term of office; they could require that of cabinet members or members of Congress. Such a law would, of course, either require presidential approval or else sufficient support in the houses of congress to override a veto.
There are various limits on what government folks can do. 18 USC 202(c) is an example of a limit on the limits:
Except as otherwise provided in such sections, the terms “officer” and
“employee” in sections 203, 205, 207 through 209, and 218 of this
title shall not include the President, the Vice President, a Member of
Congress, or a Federal judge
It is possible that a president could engage in a criminal act such as theft, and that is not permitted and would be grounds for impeachment. The president does not, however, have the power to e.g. unilaterally send all government hotel business to a certain hotel company, nor can he declare that 10% of all government expenditures must be deposited in his personal bank account, so the mechanisms whereby corrupt rules of certain other nations can get away with that is that those executives have vastly more power in their countries than POTUS does. With congressional support, though, such acts could come about. If it did, it would not be too surprising if SCOTUS ruled based on common law and considerations of justice that such a law / act was illegal, but it would not be a textualist argument.
