Scrooge was rich, and a man of business. What was his business? How did he get so rich?
Our first clue comes in the first paragraph: "...and Scrooge’s name was good upon ’Change, for anything he chose to put his hand to." That is to say, he could easily raise money on the London Stock Exchange, where bonds, commodities and other investments were also traded.
There is one reference to his warehouse. Given his wealth, this implies some wholesale element to his enterprise.
The Ghost of Christmas Yet to Come shows Scrooge a young couple who owe him a debt they cannot immediately repay, saved from ruin by his death.
For more information, we might look at Nicholas Nickleby, where Dickens gives us a more detailed view of another greedy and covetous Dickensian businessman, Ralph Nickleby. His main line of business is stock manipulation. Historically, there were plenty of 19th c. examples: Drew, Fisk and Gould will do for starters. The loose rules of the era provided opportunities to cheat even sophisticated and cynical investors like Cornelius Vanderbilt. For the naive or even average investor, so much the worse.
Ralph Nickleby, like Scrooge, also profits from moneylending. Because of the plot we know some details of one of the debts owed to him: a bit under a thousand pounds, owed by a spendthrift gentleman. It is likely that Scrooge's lending was on a similar scale: a practical man like Scrooge would much rather lend 1,000 pounds to one man than 50 each to twenty.
It would be ludicrous to claim that Scrooge's miserly nature did "a great deal of good". It simply pushed a bit more money into the already ample market of London capital seeking investment opportunities, at the cost of reducing demand for goods and services.
Some of the investment, like Ralph Nickleby's predatory stock manipulation, was probably actively bad. The purchase of previously issued shares and bonds, or existing ground rents, would only have benefitted the sellers of those assets, typically not very needy. Only when the investment financed an actual productivity improvement was the impact clearly good.
In contrast, redeemed Scrooge immediately puts money into the pocket of a street urchin, a poulterer, and a cabdriver. It did them good, surely, and penny for penny, probably more good than any of unredeemed Scrooge's careful investments.