You are right in the part of working as intermediary. But the bank (in theory) wouldn't charge me for transferring the money as the movements will be done within a same bank.
The way that I devised such lower cost was acting as intermediary between users. As transfers within a same bank are virtually "free", opening up accounts in every bank that will be accepted in the app and having enough funds in each of these banks to move money would allow me to reduce the interbank costs greatly and still make a
profit.
The diagram attached explains the idea better. User A (who has Bank X) wants to transfer $100,000 Colombian pesos to User B (who has Bank Y) through my app. The user transfers $102,000 pesos (after the 2% commission that I plan to charge) to our company's account in Bank X and we provide the service of transferring those $100,000 from our company's account in Bank Y to User B. Hope this explains the idea bette!