The present invention provides an improved billing and contract management method and a computerized enterprise resource management system for processing and management of rent-producing financial products, such as contracts and leases that are past the initial end term date, and that have not been renegotiated or extended, or, in other words, that have entered the “evergreen” period. Among other things, the method and computerized system of the present invention integrate processing of the “evergreen” contracts with a proprietary generalized ERP software product/solution such as Oracle™ E-Business suite's Lease Management system, and supplement the processing of regular contracts with the “evergreen” contract authoring and booking, processing, validation, billing, accounting, collections, modification and audit trailing of the changes made to the “evergreen” contract terms.