The founders of Skype are escalating their legal battle with eBay.

Niklas Zennstrom and Janus Friis, who became billionaires after selling Skype to eBay in 2005, filed a copyright lawsuit on Wednesday against Skype in the United States District Court of Northern California. The suit comes a little more than two weeks after eBay announced it would sell most of Skype for $1.9 billion to a consortium of investors led by the private equity firm Silver Lake Partners.

In the court filing, Joltid, a company owned by the Skype founders, claims that eBay violated copyright law by altering and sharing the peer-to-peer source code behind the free Internet calling service. The Skype founders maintained ownership of that source code after selling Skype to eBay in 2005, and licensed it to eBay.

Joltid seeks an injunction and statutory damages, which it says could total more than $75 million a day. The lawsuit also names as defendants Silver Lake Partners and its partners in the buyout, Index Ventures, Andreessen Horowitz and the Canada Pension Plan Investment Board.

Perhaps I’m missing something, but eBay spent $1.9 billion $2.6 billion to acquire Skype, yet somehow structured a deal that permitted the founders to retain the copyright to the source code?!?! This sounds absurd.

UPDATE: BusinessWeek is reporting that the lawsuit might complicate eBay’s proposed sale of Skype. Dumbasses!