Double Dipping into SaaS

Starting a SaaS is hard. Finding the right idea. The time. Following through.
What if there was a way you could get paid for work, while building a SaaS app?
What if you had a guaranteed client on day 1?
What are you suggesting?
I have a client that I know personally. They need a custom application for their business. Think a custom e-commerce shop, social media functionality, and payment system.
Integrating all of this together in a cohesive manner is exactly what they need.
It's a unique situation though. Normally I would charge an hourly or project rate for something like this. This client couldn't afford it. But there's another way.
Since I personally know this client, I opted for a different approach. No development cost up front.
I will be creating the entire system in exchange for a 3% cut of all revenue generated through the system.
It's a cool solution to multiple problems. I'm fronting all the effort. They don't have to worry about paying money and stressing over deadlines. I get to work on my own time with little pressure. I am extremely confident in the outcome and we have gone through an in-depth plan of how the system will function.
In the end, this system will handle all payments for their business. As they grow, I receive more money.
It's also more exciting for me, because I'm rewarded for the value my system is providing. If I make it better and solve their problems, there's more customers, which means more money for both of us.
What about SaaS?
On top of this custom system, I managed to find multiple SaaS ideas. I'm still early on with my ideas, so I don't want to say too much about what it does exactly.
I will be making part of their app managed through a separate SaaS app. In the beginning, the client's own app will be the only consumer. I'm building it in such a way that they will have to sign up just like anyone else would.
Pros to this approach
Day 1 I have a customer. Based on their feedback, I can make a very good MVP of the SaaS offering.
I get paid 3% the moment this system is launched.
As a side note: the SaaS application is a very small feature of their entire platform. There are no concerns about offering their app to other people, because that's not what I'm doing here.
But even if I wanted to make their entire platform a SaaS offering, I would have that ability without upsetting the client. (It just doesn't make sense, their actual platform is very specific to them.)
Finding a good relationship and a client that's okay with this arrangement can be daunting. I am suggesting that it does exist and can be done. I am not advocating for doing this in an unethical manner, however.
Win-win
Now, what if my custom SaaS idea fails? It really is a win-win scenario all around.
My client needs it and will be using it.
It will look great on my resume to show off the application and what it can do, even if it never grows.
The most likely case is that I will be able to find at least a handful of other companies to use it. Based on the pricing structure I have in mind, and the size of businesses I plan to target, I could make near 5k a month from only 3-4 clients.
Conclusion
When fleshing out ideas, try to find freelance clients. Think outside the box on how you can provide value, and how you can be compensated for your work. Try to get a 2-for-1 project where you can make part of the application into a separate system. I love the idea of having a client the moment my SaaS is finished and getting "free" testing and feedback out of it!
Even if it fails, I will end up generating some guaranteed income out of this.
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