Does the person know about the deal? If not
$25: it's enough to help the person out. Not enough to be suspicious or look like there are strings attached. Just go up to a restaurant around lunch and give them the cash to help pay for their lunch. Tell them to pay it forward sometime. Would work every time.
If they do know about the deal I would pony up half. So they won't reject out of spite.
If the amount was $10,000 that's a whole different story. I would offer $2,000. That amount is Enough that people will not reject the offer out of spite even if they do know about the deal but small enough that I still get to keep a lot of the money. No one except the really rich would ever turn down $2k. That's a nice vacation, paying off a credit card, a couple months rent, a down payment for a car. That's a lot of money to most Americans and even if they were not extremely happy about only getting 20% of the 10k most Americans would take it out of pure need for the cash.