Nan Shan Life was told to pay Tw$300,000 ($9,930) after it stripped the union spokesman of his title as a regional manager for "making comments that hurt the company's image," said the Taipei City Labour Department.
"The spokesman didn't make the comments for his personal gains. The punishment obviously targeted his role as a union member... and constituted employment discrimination," it said in a statement.
The spokesman, Lan Wei-ting, was said to have made "emotional" remarks on television and in newspapers last year, claiming the sale would hurt clients' rights and lead to lay-offs, an official at the labour department said.
Nan Shan's labour union has repeatedly voiced concerns over AIG's plan to sell Nan Shan as it tries to pay back huge loans it received from US government to save it from collapse during the 2008 financial crisis.
AIG has reportedly received five bids so far for Nan Shan, with Taiwan's Chinatrust Financial offering the highest bid of $3 billion, sources said.
Other bidders for the unit include Cathay Financial, Fubon Financial, Goldsun Group and Ruentex Group.
Goldsun has teamed up with other local business groups, including security firm Taiwan Secom and the Industrial Bank of Taiwan, to press for its bid for Nan Shan, according to the Economic Daily News.
It is also reportedly backed by Hong Kong-based Primus Financial Holdings, whose previous bid for Nan Shan of $2.15 billion was rejected by the Taiwan government last year.
Taiwan authorities cited concerns that the Hong Kong consortium of Primus and China Strategic Holdings lacked the experience needed to manage an insurer while it also failed to provide a long-term management commitment.
The rejection of the bid came as a blow to AIG, once the world's largest insurer, which has been selling assets to pay back the US government.