If you start out knowing the post ID, you could store a value in post meta with update_post_meta(). Then when the post is output in the loop use get_post_meta() to determine that value and adjust output accordingly.
I saw part of your original post in my feed reader. The above code would not affect what a user sees in their browser if they are the one who effected a change of state by purchasing something. In that case you still need to do the above server side, but additionally client side you need to update the page they are viewing. This is easiest done by reloading the page. Depending on how the payment is managed, this might happen automatically.
