How to display remaining post ( in post__in ) if posts are less then post per page?

I think the simplest, but not necessarily the most performant, would be using two WP_Query loops, one using $query->set( 'post__in', $product_ids_on_sale ); and other using $query->set( 'post__not__in', $product_ids_on_sale ); because these are mutually exclusive. One loop would be used for the main query and other for second loop as show in this example in the documentation.

You would need to account for the number of items in each loop based on the number of items in the $product_ids_on_sale variable AND pagination. And I would the main query for the products NOT in sale.