At interest vs With interest

at interest—(of money) borrowed or lent on the condition that interest is to be paid:

• The sum was placed in a bank at interest and not to be drawn before a set maturity date.

with interest—

1. (of money) paid with interest included:

• The company will return your original investment to you with interest.

2. return or repay with increase or addition:

• The boys make monkey faces at me and I return the compliment with interest.