1. Buy allowed item with necessity dollars.
  2. Merchant credited with nicety dollars.
  3. Return item to merchant.
  4. Receive refund as X% of the nicety dollars.

  1. Buy $X banana with necessity dollars.
  2. Merchant credited with nicety dollars.
  3. Give $Y rebate card to merchant.
  4. Receive rebate in nicety dollars.

Your well-intentioned scheme fails utterly at the third bullet point, because as soon as you allow the exchange to general-purpose currency, you no longer control how it is spent.