HAVANA — Sitting in her brother’s spare apartment, near a blinking plastic Christmas tree, Sonia Garro was relishing her newfound freedom, happily trading her prison garb for a purple dress and flip-flops with bright pink plastic bows over the toes.

Ms. Garro, a member of the Cuban dissident group known as the Ladies in White, had just spent, by her count, two years, nine months and 20 days behind bars. Her surprise release, a senior American official said, came as part of the secret negotiations that led to the historic agreement restoring diplomatic relations between the United States and Cuba.

But while Ms. Garro flashed a toothpaste-ad smile, thrilled to be spending Christmas with her 18-year-old daughter and other relatives, she had serious reservations about the deal. Like many dissidents, she was uneasy with the sudden rapprochement between Washington and Havana, including the softening of the longstanding economic embargo against Cuba.

“A country that violates the human rights of its people shouldn’t have sanctions lifted,” Ms. Garro said. “Here there is no freedom of speech, there is no freedom of anything. This will give them more leeway to continue operating with the same impunity that they have always operated with.”