Placement of success code in a conditional
Some notes:


Imperative programming has its use cases (or so I've heard :-)) but implementing logic is definitely not one of them. Some thoughts on the matter. Use always expressions, not statements, to describe logic; that's it, don't begin with a x = 1 and then modify x somewhere else in the code. Write expression branches instead (conditionals are in expressions in Ruby). Don't think in terms of "how" but in terms of "what".
if !x.nil? -> if x.
obj.save may fail but you are not checking it.
Don't write explicit return (more on idiomatic Ruby).
As you say the order of checks is important. I usually check for "problems" first, and keep the "ok" scenario for the last branch.
You are using a value (a hash) instead of an exception to signal errors. It's slightly not idiomatical in Ruby (in the sense that people tend not to do it, not that there's anything wrong with it), but personally I like it (and use it).


You could write:

def redeem(pin)
  success, message = if date_claimed
    [false, 'Reward already claimed']
    ...
  end
  {:success =&gt; success, :message =&gt; message}
end


However, returning {success: ..., message: ...} on each branch, yet slightly more verbose, looks pretty nice and declarative:

def redeem(pin)
  if date_claimed
    {success: false, message: "Reward already claimed"}
  elsif reward.redemption_pin != pin
    {success: false, message: "Wrong pin"}
  else 
    update_attribute(:date_claimed, Time.zone.now)
    {success: true}
  end
end