Final answer:
Expenditure of public funds is proper only when appropriated by Congress, according to Article I, Section 9, Clause 7 of the U.S. Constitution, which grants Congress the 'power of the purse.' This serves as a check on the executive branch and ensures transparency and accountability in government spending.
Step-by-step explanation:
The established rule regarding the expenditure of public funds is that it can only be done when there is an appropriation by Congress. Specifically, Article I, Section 9, Clause 7 of the U.S. Constitution provides Congress with the sole "power of the purse", requiring that no money shall be drawn from the Treasury without a law passed by Congress that appropriates the money. This clause is a critical check on presidential power, preventing the executive branch from unilaterally spending government funds without Congressional approval. Such financial oversight ensures a necessary balance of power and is reflective of the framers' intent to prevent the kinds of financial abuses they associated with monarchical rule.
In addition to this, public officials are barred from accepting titles, offices, or gifts from any foreign country without congressional approval, and a regular account of all receipts and expenditures of public money must be published, enhancing transparency and accountability in government spending. These stipulations serve to uphold the democratic foundations and fiscal responsibility central to the function of the U.S. government.