What was Ebenezer Scrooge's job in A Christmas Carol?

Ebenezer Scrooge was a
banker.

The reason Ebenezer Scrooge was so rich was that he
made his living lending other people money and charging interest.  He worked in a counting
house, and he owned the counting house because it was just him and Bob Cratchit.  A counting
house was a business for exchanging money.

The door of
Scrooge's counting-house was open that he might keep his eye upon his clerk, who in a dismal
little cell beyond, a sort of tank, was copying letters. (Stave 1)

Being a money-lender is not a very pleasant profession, because people often do not
want to or are unable to pay back the money they have borrowed.  This might be one of the
reasons why Scrooge became such a bitter man.  He saw the worst of human nature, and became
resentful.

One of the reasons that Scrooge was not willing to give away money
to the poor is that he seemed to be exploiting them in his business.  When Scrooge is visiting
in the future and asks to see emotion at a death, he unwittingly produces an image of people
celebrating his dying.

To whom will our debt be
transferred?

I don't know. But before that time we shall be ready with the
money; and even though we were not, it would be bad fortune indeed to find so merciless a
creditor in his successor. We may sleep to-night with light hearts, Caroline! (Stave
4)

The man and woman discuss how Scrooge would never
relent and give them an extension on their loan when they were unable to pay.  He had a
reputation for being hard-hearted and uncaring with his clients.  This image shows how Scrooge
affected everyday peoples lives in a negative way.

Belle accuses Scrooge of
having a golden idol, caring more about money than people.  It seems that in building his
business, Scrooge became obsessed with money and wealth at the expense of his relationships. 
This is why when we see Scrooge at the beginning of the book, he is almost all alone.  He even
rebuffs his nephews attempts at friendship because he has no money.