Imagine this is the year 2014 and you are the product manager working on building Google Pay. Can you write a Product Requirement document with core assumptions, hypothesis and use cases for the same [design your format/or pick something you already use]? You must also include user flow for P2P payment to explain the concept to the readers (tech/business team).
Product requirement document containing the following:
1. Background and objectives
2. Problem statement
3. Proposed solution
4. User stories with acceptance criteria
5. User flows
You can find good product document templates online easily. There is also tons of material on how to write good user stories and acceptance criteria.