single-page web apps + rest api combination: server or client approach?

Context

I'm building a single-page web app using AngularJS and a using REST api as backend.

The rest backend has the following services (simplification):

• /accounts/:id -- manages accounts
• /transactions/:id -- manages transactions related to accounts
• /sms/:id -- manages sending sms and listing sent sms

Problem

The problem emerges when I want to condition sms sending to the current account balance.

How to implement account balance validation while keeping the REST philosophy?

Alternative solutions

The way I see it, I have 2 alternatives:

• Server-side: sms route handles account and balance checking for sms send request (POST /sms)
• Client-side: the controller asks first for the account balance and only invokes the sms route if there is a balance

Server-side pros and cons:

• Pros: account balance checking is transparent for the web app; no account balance validation on the client-side means no chance of hacking (?)
• Cons: Implies that the sms route will be responsible also for the balance checking

Client-side pros and cons:

• Pros: keeps REST apis ortogonal
• Cons: moving validation logic to the client makes it prone to hacking

Any thoughts?