In the third paragraph of stave one, "Marley's Ghost," of Charles Dickens's novella A Christmas Carol, the reader learns that Ebenezer Scrooge "was an excellent man of business." The questions is, of course, what business?
Scrooge’s name was good upon ’Change, for anything he chose to put his hand to. (Stave I)
"'Change" refers to the Royal Exchange, which was the financial center of London during Dickens's time, where merchants, brokers, commodities dealers, and insurers met to make financial arrangements and agreements. Despite the fact that Scrooge was disliked as a person, he had a good financial reputation. This doesn't tell the reader what Scrooge did, only that he was good at it, whatever it was.
Scrooge never painted out Old Marley’s name. There it stood, years afterwards, above the warehouse door: Scrooge and Marley. The firm was known as Scrooge and Marley. (stave I)
Dickens mentions two different warehouses in A Christmas Carol. The first is the warehouse outside which the "Scrooge and Marley" sign hangs, and the second is the warehouse where Scrooge was apprenticed to Mr. Fezziwig, which Scrooge visits with the Ghost of Christmas Past in stave II, "The First of Three Spirits."
According to the Oxford English Dictionary, "warehouse" can mean a large building in which "wares" of different kinds are "housed," which is what Fezziwig's warehouse appears to be. Dickens describes Fezziwig's warehouse as large enough to accommodate a party for all of the warehouse workers and others in the neighborhood and their energetic dancing—"twenty couples at once" (stave II).
"Warehouse" can also mean a simple shop or storefront establishment, such as Scrooge's warehouse appears to be. Dickens describes Scrooge's warehouse as no more than two small rooms—Scrooge's office and "the Tank" in which a freezing Bob Cratchit labors, "copying letters"—although there might well be other rooms which Dickens doesn't mention.
Once upon a time—of all the good days in the year, on Christmas Eve—old Scrooge sat busy in his counting-house. (Stave I)
During Victorian times, a counting-house could be a bookkeeping or accounting office or department of a larger merchant business, possibly with its own rooms in a large building, or a storefront, such as Scrooge's seems to be. All of these references provide the environment in which Scrooge conducts his business, but none of them describes his actual business.
Scrooge is a miser, loner, and recluse, but he doesn't sequester himself in his counting-house unceasingly counting his money, which is the common perception of him. Scrooge regularly conducts business outside his office, which is revealed in stave IV.
Scrooge meets with merchants at the Royal Exchange, for example, where he has an "accustomed corner" where he can be found at a certain time of the day. Scrooge has also spent some time cultivating a financial relationship with two wealthy "men of business." It's possible that Scrooge manages the financial accounts of these merchants and businessmen, or perhaps he even has partnerships arrangements of some kind with them. In other words, Scrooge's business interests might be diversified.
The most specific reference to Scrooge's "business" is also found in stave IV. A man returns home to his wife, Caroline, and their children, with news that a person to whom they own money—presumably Ebenezer Scrooge, judging from the overall content of this Stave—has died. Caroline and her husband are happy to hear about Scrooge's death, and thankful to be rid of "so merciless a creditor."
In addition to whatever other business or businesses with which Scrooge is involved—the details about which he apparently shares with no one, not even Charles Dickens—he's a despised money-lender with a reputation for merciless foreclosures which ruin people's lives, and who wouldn't hesitate to invoke the Poor Laws and send debtors to debtors' prison, to the treadmills, or to the workhouses, the establishments which, as Scrooge says in stave I, "I help to support."