I'm not sure exactly what you mean. If it's a web API request from a web client that says "transfer $1,000 between account A and account B," then what choice do you have but to "trust" that number? Obviously you have to check whether the request is authenticated and is authorized to transfer between those accounts, and check if account A has $1,000, but what about the client origin of the request do you need to check?
This is the standard way online banking has been operating in, I believe, much of Europe since practically forever (although it used to be physical lists before smartphones were popular).
