You might be probably looking for a plugin that offer’s groupon-like functionality (“Groupon 1.0 started on a WordPress blog.”).
By googling I learned that there is a plugin called WP Coupon (WordPress Plugin) (review). No idea if it fits in what you need, but it looks like it does what you’re looking for.
But it’s probably additionally worth to checkout if groupon and other similar fast growing sites do offer a webservice so that actually the job is already done: