Ford Contour 96 timing belt changed and now pulley

Hi , I got my car's timing belt changed less than a month back and now the mechanic is saying that the problem is in the pulley that holds the belt. I am worried becoz i am thinking that can it happen that mechanic
didn't see the bad pulley while putting the new belt in? He is chargind me another $400 for the new pulley.

Add pictures here
<% if( /^image/.test(type) ){ %>
<% } %>
Add image file
Upload is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here. All logos and trade names are the property of their respective owners.