Most tourist guides dismiss the importance of the Moneda Nacional currency and recommend that foreigners simply stick to using the CUC currency. If you are staying at an all inclusive resort or plan to spend all of your time in touristy areas, then this is fine. If, on the other hand, you are planning to explore the island or visit non-touristy areas, it will be much more economical to use the Moneda Nacional currency for most of your expenditures.
The CUC currency is used in tourist resorts, high end hotels, fancy restaurants and to rent apartments. Furthermore, it is used in large state-run supermarkets and stores and to take certain types of taxis. While in some situations, you must pay for your goods and services in CUC, there are many other instances where you can shop at locations which sell identical products, at a far lower price, in MN currency.
Moneda Nacional is sometimes called CUP.
Basically, you should have CUC and MN in your wallet and know how to use them. 1 CUC is worth 24 MN. 1 peso Moneda Nacional is worth about 4 cents CUC (equivalent to 4 cents USD). If you don’t know how to use the money or if you do not insist on getting change back, you usually will not get change back on your purcahse… You will simply be scammed and the server will keep all the change.
For 99% of street food purchases, you can and should pay in MN. Basically anywhere that accepts payment in MN, you should pay in MN. The reason for this is that most places generally do not carry or offer change in both currencies. And if you hand over a huge bill to pay for a small purchase, than the storekeeper will instantly know that you are a foreigner and they will in most cases try to rip you off by giving incorrect change.
For example: There is a sandwich on sale for 5MN and you want to buy it. You offer to pay with a 3CUC bill. The correct change on your purchase would be 2CUC plus 19MN, or a total of 67 MN (1cuc = 24mn).
In this situation, the most likely outcome is that the storekeeper does not offer you any change at all. He just gives you the sandwich and say “have a nice day.” If you insist on change, he might offer you a 10 peso MN bill or maybe a 20 peso MN bill. Or maybe he just reaches into his pocket and hands you a 25 cents CUC coin, or a 50 cents CUC coin. A token amount of change just to make you go away. You will never get exact change. Even a Cuban local would not get exact change on this transaction. The local would probably get about 65MN in change, if he is lucky.
The best and easiest way to pay for small purchases is with small denomination bills. If the sandwich costs 5MN, then pay with 5MN. Or pay with 10MN and you will always get 5MN back as change. Always keep things simple.