In Charles Dickens's novel A Christmas Carol, what is meant by the phrase, in the opening paragraph, that "Scrooge's name was good upon [the exchange],...

This
is an interesting one, linguistically speaking. If you look in the Oxford English Dictionary,
you will find under the word "change" several definitions, one of which is "a
place where merchants meet for the transaction of business." The entry also notes that the
word was therefore also sometimes treated (incorrectly) as if it were a shortening of
"exchange"hence the apostrophe to indicate elision at the beginning of this word in
Dickens's text: "'Change." But in fact, Dickens may be using a common idiomatic phrase
which need not refer specifically to the London Stock Exchangeas has been suggestedat all. By
saying that Scrooge's word was good "upon 'Change," Dickens may simply be saying that
Scrooge was well known as being a man of his word, a man who would never come to a place of
business and fail to have the right funds or to deliver upon a transaction. If we understand the
phrase to be idiomatic in meaning, or allegorical, we can see that it might be used more widely
to suggest that someone was a man of his word and of repute, whether or not he was
rich.

More specifically, of course, Scrooge is rich, and
Dickens is playing on this fact here. He is saying that, because Scrooge is known to be a man of
his word (and in this case, a man who is also known to have a lot of money), anything he says is
to be believed, and any contract he signs would have value. So, effectively, because Scrooge has
signed the register of burial for Marley, it must be construed as absolutely definite that
Marley is dead, because Scrooge always keeps his word and does not sign false
contracts.