USB Switzerland ping
From the link: “According to Cuban government figures, its revenues from tourism and hard-currency stores — where goods are sold for U.S. dollars, mostly to Cubans who receive remittances from abroad — amounted in 2002-03 alone to about $2 billion.”
Dollars entering Cuba don't accumulate. Cuba has imported food and agricultural products from the U.S. since 2000 and must pay cash on delivery. This totals more than $4 billion. They buy U.S. pharmacuticals and other U.S. products through third parties who often require U.S. currency.
Cuba has long been a transhipment site for South American narcotics entering the U.S. These services could account for the USB money as the Castro brothers would want to keep these receipts off the books.