How to use a PIN Code Verification for redeeming Coupons?

How to make a PIN Code required to redeem Coupons in Order to only allow Coupon Redemptions verified by the Sales Staff?

Customer clicks on Redeem Button

  1. The customer opens the mobile coupon on his mobile phone
  2. The customer clicks on the Redeem button to activate the coupon
  3. After the customer clicked the redemption button, he is asked to enter a PIN code.

Sales Staff says PIN Code

  1. The sales staff tells the PIN code to the customer
    1. One PIN: It's possible that you have only one PIN for all coupons
    2. PIN per store: It's possible that each store has it's own PIN for all coupons
    3. PIN per coupon: It's possible that each coupon has its own individual PIN

Customer enters PIN Code

  1. The customer enters the PIN-code on the mobile coupon and confirms
  2. The coupon gets redeemed and the coupon code appears