The Kendall case is about contract law, which is far different than Congress compelling the Executive Branch to spend money it has appropriated. Congress simply does not have that right.
No, it's not. Because Congress had passed an act requiring the Postmaster to give the contractor the credit he sought. As a result, the act of granting the credit - the same thing as spending federal money - was a ministerial act that the executive branch could not refuse to fulfill.
In other words, under Kendall, the Supreme Court directly held that Congress can, by suitable legislation, compel the executive to spend federal funds.
As the Court expressly stated, to hold otherwise would be to give the President the power of dispensation, to become a law unto himself regardless of what Congress may have lawfully enacted.
If Congress gives the president discretion, then yes, the president has the authority to withhold funds within the parameters of the discretion, but if Congress passes a law that says "The President shall spend $X on Activity Y", then then president has absolutely no discretion in the matter and must spend that money as directed by Congress.
To hold otherwise would be to give the President the power to do indirectly what the President cannot do directly.
The President's power over what is, and what is not, law is through the veto power - if the President does not want something to become law, then he can veto the act - however, the Constitution expressly provides that the president's veto can be overridden by Congress. Thus, to then find some power to impound - just another emanation from auras and penumbrae - is to allow the president to nonetheless frustrate the enacted will of Congress through the simple expedient of refusing to spend money to carry out the commands of Congress as set out in duly enacted legislation.
To be perfectly honest, if one really believes that the emanations and penumbrae of the Constitution give the president a free-floating power of impoundment, then one is committed to the position that Biden was perfectly within his rights to refuse to enforce Congressionally enacted immigration law - after all, he was simply refusing to spend money to carry out that legislation, and this free-floating view of impoundment means that Biden was acting within his Constitutional authority.