production order quantity should not be greater than planned order quantity

Dear Gurus

Suppose I have got a sales order of 100 pcs. I ran MRP and planned order of 100 is generated.Now my client is saying that while converting this planned order to production order the system should allow him to enter a quantity less than 100 but the system should not allow him to enter more than 100.That means they dont want to produce more

Regards

Sandip Sarkar