Select the most appropriate one-word substitution for the given group of words. 

An act of getting money back from people it has been paid to.

Refund

The most appropriate one-word substitution for the given group of words is refund.

• Retrieve means to get something back that you have lost or had taken away from you.
• Cashback means a partial refund of the amount spent on a purchase.
• Retreat means to move back or withdraw from a place or situation.

Of these words, only refund specifically refers to getting money back. This is the most common way to refer to the act of getting money back from people it has been paid to.