For Brits, like Dickens, it would have been the same thing; sterling was on the gold standard at the time.
I think the point is that the US dollar used to be freely convertible into gold at a fixed rate, but this was suspended during and for some time after the civil war, and at this time if you converted dollars into gold (or any gold standard currency) you would get rather less gold/currency than you would have done a few years previously.
It’s not strictly correct to say that Dickens (or anyone else) lost money when selling dollars. They ended up with less money than they would have done, had they sold a similar quantity of dollars a few years earlier, but as they didn’t have a similar quantity of dollars a few years earlier that isn’t really a loss. Still, at the time a lot of people would have felt that the previously-fixed conversion rate represented the natural or true value of the dollar, and that if you sold at less than that rate you were in some sense losing money.