Anyway to stub local variables in an rspec test?

I have code that looks like this in a method that I am trying to test:

  service = if user_activator?
                UserActivationService.new(subscription.user, cookies: cookies)
             elsif sub_activator?
                 SubscriptionActivationService.build(subscription: subscription, cookies: cookies)
            end

I am trying to stub service to be an instance of UserActivationService and then try to see if it receives a method with some code that looks like this:

expect(user_activation_service_instance).to receive(:activate).with(hash_including(
          app_context: app_context,
          cookies: cookies,
          subscription: subscription
        ))

But is there a way to stub a local variable to be an instance of UserActivationService??

Here are some ways that I think may work?

First, should I extract that conditional local variable set into a method like this (this feels fishy):

def service
    if user_activator?
      UserActivationService.new(subscription.user, cookies: cookies)
    elsif sub_activator?
      SubscriptionActivationService.build(subscription: subscription, cookies: cookies)
    end
  end

then I can do:

let(:user_activation_service_instance) { UserActivationService.new(subscription.user, cookies: cookies) }
allow(<something that returns an instance of the class I am testing>).to receive(:service) { user_activation_service_instance }

then

  expect(user_activation_service_instance).to receive(:activate).with(hash_including(
          app_context: app_context,
          cookies: cookies,
          subscription: subscription
        ))

Is this kosher?