Hoang Ngoc Viet, vice director of the provincial Department of Agriculture and Rural Development, said they freed the animal right after fisherman Hoang Luong handed it over to them.
Immediate action was required because upcoming rains could pose a risk to the mammal at Tam Giang Lagoon, the seal's habitat, said Viet.
Most local people were notified about the release to encourage them not to catch rare wild animals, Viet said on the news website.
The aquatic mammal weighing some 30 kilograms was caught in Luong’s net while he was fishing on Tam Giang Lagoon, the fisherman told the press.
He planned to sell the seal to the highest bidder and refused to give it to management agencies despite their requests that he do so.
On Thursday, when the provincial environmental police and the agriculture department’s officials came to persuade him to hand over the seal, his wife objected and then fainted, prompting some locals to throw stones at the officials' vehicles, the news source said.
The next day, Luong agreed to hand it over to them on condition that he would receive financial reward for his efforts and expenses for taking care of it, according to Viet.
The total will amount to less than Luong's requested sum of VND15 million (US$769.62), Viet told VnExpress.
A fisherman in Thua Thien-Hue captured a seal on Tuesday, and plans to sell the animal to the highest bidder.