1.) The only time it’s a big hassle for Cubans to exchange their foreign currency tips is when they receive coins because the bank/Cadeca won’t accept them. People, leave those Toonies, Loonies, 1 pound coins, etc. at home!
2.) The only time it’s a slight hassle for Cubans to exchange their paper foreign currency tips is sometimes they’re not allowed to use the Cadeca at work, they have to exchange at a bank/Cadeca elsewhere.
Whether it’s a scam or not is simple math… look at the exchange rate and see if what they’re requesting is fair. I’m betting that almost undoubtedly they’ll be making a few percentage points at your expense…