I'd phone the Marriott again and talk to a manager. If that fails, do a credit card challenge.

This happened to me with the Westin Grand in Berlin. They took care of it when I pointed the problem out, and it took about a week for the charge to be reversed. When it was reversed, I had lost money due to the currency conversion, so they charged back another 15 Euros.

This has otherwise not happened to me with Priceline, but it did occur at another place where I had a free night.