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n. a specific amount stated in a contract or negotiable instrument (like a promissory note) at the time the document is written. A sum certain does not require future calculation or the awaiting of future happenings. Example: "Wanda Williams will pay Wilma Jackson $10,000 for redecorating her house, including all costs," is a sum certain as compared to "Williams will pay Jackson for time (at the rate of $50 an hour) and costs of materials for redecoration of Williams' house."