Passing Real Time update to Stripe Checkout Amount

I am trying to implement a variable price in the Stripe Checkout UI that will be updated by Pusher.

I begin by assigning the Pusher message to a global variable called price.

  <script type="text/javascript">
      var pusher = new Pusher('7a5433c6fc39502a4a02');
      var channel = pusher.subscribe('the_channel');
      var price
      channel.bind('the_event', function(data) {
      price = data.message
      });
  </script>

If I assign price to data-amount, I am then confronted with two problems:

1- process-wise, the field would be empty by default, populating only when Pusher sends a message.

2- syntax-wise, the variable currently does not even render the message when pushed

<%= form_tag charges_path, class: 'stripeform' do %>
    <script src="http://ift.tt/1doUtf9" class="stripe-button"
            data-key="<%= Rails.configuration.stripe[:publishable_key] %>"
            data-amount= <%= @price %>
<% end %>

What to do?