Deuteronomy 25:13
You shall not have two differing weights in your bag, one heavy and one light.
What it says
A person must not carry differing weights in the bag, one heavy and one light.
What it is doing
It forbids hidden manipulation in buying and selling.
In context
13You shall not have two differing weights in your bag, one heavy and one light. 14You shall not have two differing measures in your house, one large and one small. 15You must maintain accurate and honest weights and measures, so that you may live long in the land that the LORD your God is giving you. 16For everyone who behaves dishonestly in regard to these things is detestable to the LORD your God.