A German man made the surprise discovery of a large bundle of banknotes in an old clock he purchased at a flea market, in a story first reported by German broadcaster NDR. The only problem was, the banknotes were of a kind no shop or restaurant would accept — Deutsche Marks, the currency of Germany until 2001.

He only discovered the hidden treasure trove of 50,000 Marks in the wooden paneling of the clock when he got it home from the flea market. The man, a resident of Aurich in Lower Saxony, took the money to the local government lost-and-found office.

Read more: The never-ending story of Germany's ghost money

After a legally-required waiting period of six months, during which no one claimed the decades-old banknotes, the man was permitted to have them back.

It is still possible to exchange Deutsche Marks (commonly known in English as Deutschmarks and in German as D-Marks or Marks) for euros at Germany's central bank, even though the currency has not been used for 18 years.

This is exactly what the man did, although he also had to pay an administrative fee of €576 ($639) to the city of Aurich.

How much is 50,000 D-Marks worth?

The official exchange rate between Euros and Marks has remained unchanged since 2001: One Euro is worth 1.95583 Marks.

This would place the value of the man's find at around €25,500 ($28,400).

Germany's cash-centric economy and culture of saving means that there are often stories of cash being found in people's houses, sometimes having been stored for many years. The central bank estimated in 2018 that there were still over 12.6 billion Deutsche Marks (around €6.3 billion) in cash not accounted for.

A large number of these are most likely in coins lost behind sofas and small bank notes kept as momentoes and not really worth a trip to the bank. But there remain, no doubt, a considerable number of mini-treasure troves like this one discovered in north Germany.

And time may be running out to cash in these hoards. Some other eurozone countries no longer allow exchanges to euros at all: French francs, Italian lire and Greek drachma are all worthless in 2019.

It is unclear whether the current situation in Germany will or can continue indefinitely. Nevertheless, if you make a discovery like this, it's certainly not worth wasting any time before heading down to the German central bank. The exchange rate is fixed, but the period has never been specified and could one day end.