In Belarus, the government doesn’t allow trading of its ruble outside a narrow price range, which greatly overvalues the ruble— so there is a price floor on the ruble compared to the euro or dollar. Because of the floor, currency trading had dried up: who would want to sell foreign currencies for grossly overpriced Belarussian rubles??
A friend of one of my students has a website designed to overcome rigidities in this market, sort of a Craigslist for currency. People specify amounts willing to buy or sell, agree to trade at some price and arrange a meeting place (often one of the empty currency-trading booths!). When they meet, trade nominally occurs at the official price floor, making the transaction nominally legal; but the person selling rubles makes side payments to the buyer to lower the price sufficiently so that the trade actually takes place at the equilibrium price.
One more way in which technology helps markets circumvent imperfections and rigidities.
(HT to MK)