As you perhaps already know, in Cuba, they use the Cuban dual currency CUC and CUP. Both of them aren’t international traded.
For tourist is confusing the fact to be used to two different currencies. Its easy get CUP mixed up with CUC and bad intentioned persons can take the advantage to cash you more for something. give you the money exchange back in CUP instad of CUC.
Actually 1 CUC = 1$ = 27 CUP. With food its quite easy to do a mistake, for example one buger on Old Havana can cost 6CUP (0.25$ aprox.) but in Europe and USA a hamburger can cost 6$ so its easy to pay 6CUC by mistake.
How to avoid Cuban dual currency CUC and CUP scam?
Don’t be in hurry, take your time to get used to this two currencies. Ask to the owner of your “Renta” or Hotel to explain you the differences.
Pick the calculator always with you and don’t be afraid to use when you gonna buy something.
If you gonna travel to Cuba, check also this post about original Cohiba seller scam.