Iowa Republican Sen. Charles Grassley is “very concerned” about a provision in the financial overhaul bill designed to pay for the legislation, an aide said Thursday, potentially complicating White House efforts to build a filibuster-proof majority to back the measure. If Grassley decides to vote against the bill, Democrats would be left with little margin for error when they bring the bill to the Senate floor, which could happen as soon as next week. Grassley was one of four Republicans to support an earlier version of the bill when it narrowly passed the Senate in May.