What the hell are you trying to prove?
They went back out and this time told people that the dollar/pound/peso was going to charity (THIS IS NOT A PIVOT). Sure enough they came back with 50 sales (they gave the money back each time) and told us that they proved demand for the app. Of course we said no, you proved that people were willing to give money to charity and that you are good salespeople.
You may think that you would never make the same mistake, but it happens over and over again in initial validation stages where teams try to come up with an incentive (gamification, contests etc) to take that coveted call to ACTION (again, in this case, reaching into their pocket with the intent of paying one $/E for an app that helps you meet people at an event).
Incentives, games, and other hacks may work as a growth strategy. It is not a validation strategy. If they need an incentive, they are not an early adoptor.
Remove every variable you can that does not directly prove value in the actual product or service you are offering.