A sum certain is a specified and set amount of money owed by one person to another. It is a legal term of art, having specialized meaning in the law. Some kinds of legal claims can not be brought at all unless the sum certain can be plead. A document claimed to be a negotiable instrument

Negotiable instrument

A negotiable instrument is a document guaranteeing the payment of a specific amount of money, either on demand, or at a set time. According to the Section 13 of the Negotiable Instruments Act, 1881 in India, a negotiable instrument means a promissory note, bill of exchange or cheque payable either...

can not be negotiated unless it is for a sum certain.