It's not 'incredible' idea)) It's very simple, but no one has implemented it before. It's not a debit card with a cashback, it's a mobile application.
Sure. No one has implemented a mobile application before? 
The difference between them is that the percentage returned by the buyer reaches 30-50%. The money returned by the partners of the service (this is about 10%) is paid to those who made purchases earlier, usually 2-4 weeks ago. The project is not a ponzi scheme. Users do not pay a money, but make everyday purchases - these are popular products in grocery stores, cafes and more.
All money received only from service partners (this is how standard cashback works). The service will be able to work successfully if there are 10,000 users (this is not much).
This is a very rough description. Will be gamefication, drawing of prizes between active users, referral program.
Seems like you know what you're doing. Your best bet is finding investors in your local circle or one of the start-up funding sites. Also, $20,000 is something a bank can easily sponsor as a loan. Why don't you try that? Everything I mentioned before has more chances of getting you an investment over bitcointalk. Also, if you have to mention your project is "not a ponzi" then it makes it even harder to find any support on bitcointalk. Good luckAll money received only from service partners (this is how standard cashback works). The service will be able to work successfully if there are 10,000 users (this is not much).
This is a very rough description. Will be gamefication, drawing of prizes between active users, referral program.