Peculiar Discounts posting

Hi,

I was having an opinion that discounts given to customers at the time of invoicing (not the payment terms discounts) will not have any accounting postings because the invoice amount can be recorded net discounts.

However my client is claiming that its not true because the discounts given at the time of invoicing can be posted as follows :-

Customer : Debit 900

Sales : Credit 1000

Sales Discount : Debit 100

Can any accounting gurus confirm the above postings are correct to record discounts given at the time of invoice? To me it looks a bit weird because the sales figures are overstated including of the discounts. Is this right or wrong?