Monkey = £500 or £50,000

£500 pound is a Monkey because an old English note of that value had a picture of a Monkey on it.

Why in Pound sterling is £500 sometimes called a monkey?

My Dictionary of Slang says "c19. origin unknown". Pity.
It also says that the term applies to dollars. Is this true?

I have never heard it used here in the NE part of the US of A, which by the way, is now being run totally by the extreme right-wing factions of the Republican Party.
The corporations won yesterday and the people lost big time!
Anyway, I have heard of monkey bars, monkey suit, monkey business, grease monkey and monkey on my back but never in reference to money.
Maybe some other part of Deutschland, whoops, sorry, I mean the US of A, has monkey as a reference to money.

Eric Partridge's A Dictionary of Slang and Unconventional English has this for "monkey":

£500 (in U.S. $500): 1856, 'The Druid'; Whyte Melville. (The O.E.D. cites an 1832 text in which, prob. erroneously, it = £50.) Among stockbrokers, however, however, 'monkey' (in C. 20) = £50,000 of stock, i.e. 500 shares of £100. Cf. 'pony'.

(For "pony," Partridge has £25 in general use and £25,000 of stock in brokers' use.)