This reminds me of a scenario I used to get often in class:

I bought a used construction trailer worth $50,000. I only paid $35,000 for it. Why can't I enter it as my $50,000 asset? Answer: you didn't pay $50,000 for it. If it really was worth that, why didn't you Pay That?

The owner cannot bring a physical thing they just bought for $80 to the office and get reimbursed for $100. It's the same concept.

We are not making change here. We bought something at a cost, not what it Could have cost or Would have cost. The coupon is not worth $100. It's clearly worth $80 of value. Or, if that is 20% discount, then it's value is 80% of whatever they buy, the same as shopping on the Sales Day.

I just love how people intend to leverage anything that occurs to them.

If the owner wants to be reimbursed $100, let them buy something for $100.