Concurrency problem.

Question

Dear friends, Please help me for the following problem. We have developed a web application in .net which allows user to buy refill cards online. Every refill card has pin codes attached to it. One refill card can have several pin codes. This pincode is sent to user in his mail of order confirmation. It is highly important that every user should get unique pincodes. Actually there is databse table which has refill card id, pincode, and one field which indicates whether the pincode is sold or not?(Y/N) We are showing refill cards whose pin codes are not sold i.e. soldstatus=N When he purchases a refill card with some x quantity then database should lock x pincodes for this refill card and when he confirms the order the soldstatus of all t hese records should be updated. But the problem is if he does not confirm order then these refill card should be available to purchase to other users. We can not update status of refill card unless the order is confirmed. If multiple users purchases then uniqueness of pincodes should also be maintained. This is basically a concurrency problem. Please help me. we are doing it in .net. regards. amol