SAN FRANCISCO — After collecting more than $104 million from the Bay Area Toll ­Authority in return for terminating a swap agreement, Ambac Financial Services LLC sued the agency this week seeking an additional $50 million.

Oakland-based BATA is a government agency that operates seven state-owned toll bridges in the San Francisco Bay Area. In July, it terminated $1.1 billion of swaps with Ambac Financial, a unit of bond insurer Ambac ­Financial Group Inc., because the bond insurer’s credit ratings had fallen to junk status.

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