The consequences of returning an item after receiving it

Hello madam,

A sales agent recently came into our house (around 8 pm) to do a demonstration of an expensive vacuum cleaner. After the demo, my wife and I decided to take 1 unit because they have offered an installment payment scheme for 12 months.

The following day, we talked to the agent that we will not proceed with the purchase and he just said that we will go to jail because of the law and there is no way to return the item because the sales contract includes that a customer will not be able to return the item once it is received.

As of now, we haven't paid anything to them as they first require us to pay by cheque. but then he requires us to pay cash in the first month.