If he has sketches and money in his pocket, then presumably the sketch is not money but something else.

In French "monnaie" means coins or loose change: could "money" have had a similar meaning in the English of the 1780s and 90s? In which case it might not have been too fanciful to call a banknote of the time a "sketch", as it was clearly not a proper painting - the first coloured banknotes were not issued until 1928, and were also the first notes to be printed on both sides.