San Francisco-eWork Exchange, a Web site designed to link free-lancers and corporations, said Tuesday it will buy purchasing, reporting and payroll services company TriStep, Fremont, Calif., for $40 million. The move will extend eWork's capabilities. It will move from listing corporate projects available to free-lancers toward a system that also bills the hiring company and ensures the free-lancer makes good on the contract. Conversely, free-lancers using the system will be able to run aspects of their business such as invoicing, health benefits and retirement programs through the site. eWork, San Francisco, is challenging such companies as Monster.com for supremacy in the Web employment market. It doesn't charge for listings, but collects a percentage of all work hired through its system. Since its September launch, eWork has listed an estimated $3 billion in work-for-hire on the site. Details of the $40 million transaction were not disclosed.