Republican lawmakers, with little public debate, quietly added a billion-dollar health-care benefit to legislation that was rushed through Congress just before it adjourned Saturday morning. Acting at the urging of several major business lobbies eager to reduce their medical-insurance costs and the outgoing chairman of the House's tax-writing committee, lawmakers adopted the provision even though only a single committee had previously approved it. The provision, which materialized without fanfare late last week inside a massive tax-cut measure, expands the amount of money that can be contributed tax-free to health savings accounts (HSAs). The accounts can be used to pay medical...