You're forgetting about the potential gains of the original $100 @Ik0n88x The $100 could've been invested at say 8% or it could've been used to back a horse at 100 to 1?
Really.......................$100.00 The original $100 from the register. BUT returned! Therefore $0.00 He literally gave her $70.00 in unpaid merchandise. Then he gave her an additional $30.00 cash.