Warren Buffett’s Berkshire Hathaway has joined Goldman Sachs in the investment bank’s bid to buy $3bn in tax credits from government-owned mortgage giant Fannie Mae, reports the WSJ. The report comes amid news of Buffett’s $26.6bn purchase of US railroad operator BNSF. Buffett’s involvement adds a twist to an already politically sensitive deal, as the US Treasury considers blocking any potential sale on the grounds it would not benefit US taxpayers. Companies such as Goldman and Berkshire could use the credits to offset federal tax expenses.