Qitzur Shulchan Arukh 62:2
ב: כשם שיש אסור אונאה במשא ומתן, כך יש אסור אונאה בשכירות ובקבלנות ובחילוף מטבע
Just as there is a prohibition of cheating in buying and selling, so too there is a prohibition of cheating in hiring, contract work, or money exchange.
I think this required spelling out because of the definition of ona’ah. By default, the relevant value of a business deal is the value of the item. When we speak of unlawfully overcharging or underpaying, we mean by more than 1/6 of the item’s market value.
This applies to goods. Extending it to services, which are not objects holding inherent value, required an explicit statement.
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