It looks like this translation isn't quite consistent in its usage of Russian vs British currency units. There are a few cases where farthing is used in a single sentence with rouble:
“I have not got a ten-rouble note,” said the prince; “but here is a
twenty-five. Change it and give me back the fifteen, or I shall be left
without a farthing myself.”
Now, farthing in this context doesn't mean a specific amount of money, but rather is used as an idiom for a tiny sum. However, in a few other places the translator uses copeck to express the same concept:
shall require both clothes and coat very soon. As for money, I have
hardly a copeck about me at this moment.
And sometimes a penny, or its derivatives:
“You are a good fellow, but very silly. One gives you a
halfpenny, and you are as grateful as though one had saved your life.
He came up to me and said, ‘Buy
my silver cross, sir! You shall have it for fourpence — it’s real
There seems to be no system behind these inconsistencies. So the answer is plain sloppiness on part of the translator and editor(s), if any existed.