To be even more specific, I would be extremely curious to hear of any domestic opposition to the Marshall Plan apart from that of American Soviet sympathizers.

The United States Agency for International Development (USAID) deals with most of the civilian foreign aid programs that the US conducts. They take guidance from the President, State Department, and National Security Agency on how to distribute aid to other countries.

There is a distinction between military and civilian aid. The President,State Department, Intelligence Agencies, and the Department of Defense handle military aid to our allies.

Prior to the 1961 Foreign Assistance Act, aid to foreign countries was handled by individual government agencies who developed their own programs, like the "Food for Peace" program of the Department of Agriculture. The Foreign Assistance Act put all US foreign aid under the umbrella of the USAID.

I can't answer your question directly, but I will try to provide some context, if this breaks any rules let me know.

The idea of using money to subsidize other governments, especially in terms of military aid (which is a big chunk of foreign aid even now) has a long history. I can't tell you when it started but I do know that subsidies to allied states were a big part of international interactions in the 17th and 18th century.

Foreign aid in this sense, would only be controversial as far as the political purpose to which it is being put is controversial. The idea that you send money to states to support them, or get them to act in your interest is not a modern idea.

(source: Tim Blanning's The Pursuit of Glory)

I should note that this primarily applies to military aid in the form of subsidies rather than the modern context of providing disaster relief or direct aid to governments to finance normal expenditures.

[Late edit to fix some stupid spelling errors]