omidgs on "custom fields display similar amounts"

hello every one
i use this method to display cutum field in my theme
at the first add this lines to the firs of pages
<?php $buy = get_post_meta($post->ID, 'buy', true); ?>
<?php $gheymat = get_post_meta($post->ID, 'gheymat', true); ?>
and next to disply values i insert this line
<?php echo $buy; ?>
<?php echo $gheymat; ?>
in the single.php all things are good and working fine
but in index.php and category.php all amounts are similar
for example if good 1 is 200$ and good 2 is 100$ all goods buy show 100$
(the newest amount
)
whats problem?
please help

the page code

<?php $buy = get_post_meta($post->ID, 'buy', true); ?>
<?php $gheymat = get_post_meta($post->ID, 'gheymat', true); ?>
<div class="container">

	 <div class="posttitle">
    <ul class="jDesign_tabs">

	<div class="shop"></div>
        <li><a href="#tab2" title="برای دیدن محصولات فروشگاه کلیک کنید">تازه های فروشگاه</a></li>

    </ul>	

    </div>
    <div 

class="jDesign_ir">

        <div id="tab2" class="jD_content">
          	<?php if (have_posts()) : ?>
        <?php while (have_posts()) : the_post(); ?>
			<div class="shopbg">
            <a href="<?php the_permalink() ?>" rel="bookmark"></a>
        <a href="<?php the_permalink() ?>"><h2 class="stitle"><?php the_title(); ?></h2></a><p class="gh"><?php echo $gheymat; ?></p>
        <a href="<?php the_permalink() ?>" rel="bookmark">
</a><a href="<?php the_permalink() ?>"><div class="sthumb"><?php the_post_thumbnail( array(150,150) ); ?></div></a>
					<a class="buya" href="<?php echo $buy; ?>"><div class="buy"></div>خرید پستی</a>
                    <a class="buya" href="<?php the_permalink() ?>" rel="bookmark"><div class="smore"></div>توضیحات
</a>
            </div>	

       <?php endwhile; ?>
<?php else : ?>
 <?php _e('Not Found'); ?>
?php _e('Sorry, no posts matched your criteria.'); ?>
   <?php endif; ?>

    <div class="sp2"></div>
     <div class="pages"><?php
 SEO_pager();
?>	</div>
        </div>

</div>
</div>