Look, you have a payment processor A and implement it in website of business B. You take a share of all the trades, have I understood that correctly? And you didn't tell either company A or B about it? I think that is probably stealing money. Don't wonder if one side figures that out that they sue you.
If your small business goes to Stripe/Square/etc you will be quoted a 3% + 30cents transaction fee. By pushing volume through them, they lower your transaction fee. As far down as 2% + 0cents. This would allow you to add value while profiting from the spread.
And to your stealing money comment, all business is stealing money. You take a service that is less than you sell it for. If you were doing it any other way, you wouldn't be in business for long.
