I'll watch for the answer to this. I tend to keep my pocket cash after trips, partly in hopes I'll return somewhere enjoyable, but have been caught with old notes that get hard to exchange.

In London, I disposed of old paper pound notes (would've required a trip to a particular, inconvenient branch of the Bank of England to swap) by dropping them into the donation bin at the British Museum. I figured it was still charitable, and less hassle for a large organization to deposit than a foreigner to exchange.

I had like £‎‎20-40, though, not 1000. (Only the former is in line with my usual spot donations to a charity. I'd be planning my giving at the latter rate.)