It is a revenue stamp for a 'Hundi' (note 'HUNDI' at the top of the stamp) - an Indian bill of exchange. A bill of exchange is a written note given by one person to another, instructing a third person to pay whoever presents it to them a sum of money.
A more detailed explanation according to Wikipedia:
"
A hundi or hundee is a financial instrument that was developed in Medieval India for use in trade and credit transactions. Hundis are used as a form of remittance instrument to transfer money from place to place, as a form of credit instrument or IOU to borrow money and as a bill of exchange in trade transactions. The Reserve Bank of India describes the hundi as "an unconditional order in writing made by a person directing another to pay a certain sum of money to a person named in the order.""
Another one here:
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