When merchants bought goods, instead of paying for them with gold or silver, they simply filled in a piece of paper called the which ordered the goldsmith or silversmith to give a certain amount of the precious metal to the person who sold the goods.

The answer is bill of exchange.

Explanation:

A bill of exchange is a written agreement between a buyer and a seller. It compels the customer to give a certain amount of money (or precious metal) for the goods. A bill of exhange might fix a period for the buyer to pay the debt, and it must be signed by the buyer to be valid.  

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