Designing wallet system in nodejs

I am making a product for which I want to have a credit-based system so that a user can buy credit with real money and use it within the app. I am using Nodejs for the backend and MongoDB as the database. Some approaches I've looked into:-

1. Write models for credit system and use mongo transactions throughout.
2. Use blockchain for making a new closed(only for my app) currency.

I don't know which way to go or even if one way is wrong or not. Please share some ideas if you have and maybe some open source libraries that can help.