WASHINGTON—Democrats took a step toward their goal of overhauling financial regulation, reaching a tentative deal to set restrictions on trading in exotic financial instruments known as derivatives.

Among the considerations still in the balance: A big provision being sought by Warren Buffett in recent weeks. A key Senate committee had changed its proposed overhaul of derivatives regulation after lobbying by Mr. Buffett's Berkshire Hathaway Inc., potentially helping the famed investor avoid a financial hit, congressional aides...