The estate of Lehman Brothers, the bankrupt investment bank, has settled a $6.1 billion dispute with a unit of Ambac Assurance, the bond insurer, with both parties agreeing to drop claims against each other. It is the second large claim settled by Lehman with a derivatives counterparty in recent weeks, as the estate continues an effort to settle such disputes and avoid costly litigation in a bankruptcy whose payments to lawyers and professionals is already approaching $2 billion worldwide. Ambac had filed a $6.1 billion claim against Lehman in 2009, arguing that Lehman owed money relating to the settlement of credit protection that Ambac had sold to Lehman on packages of residential mortgages. Lehman claimed that after Ambac’s bad assets were taken over by regulators in the state of Wisconsin in March, it was entitled to payments from Ambac relating to the termination of those contracts.