“...Congress of expending, on objects of benevolence, the money of their constituents.”
Your quote does not seem to apply; how is Congress “expending” anything at all when all it does is require an individual to lay aside money for their own old age so that they do not become a public charge when they are too old to work?
Anyone who thinks that Social Security constitutes the "laying aside" of anything at this point in history is naive beyond words.
It is not a "trust fund" or a savings account. The money is taken from the people forcefully, put into the treasury, and promptly spent on current programs.
But, having said that, where is the Enumerated Power in the Constitution to force anyone to lay aside anything? Because, even after reading the Constitution many, many times, I just can't lay my finger on such a power.