Manually adding discounts in Sales order

Hi -

The customer doesn't want to change the Item price list ..But want to Freely insert any discounts [On the grand amount] based on no rules..just as he likes ...

Could that be done through sales order ? or it should be handled through invoicing ?

On other hand ..can i define a discount modifier based on the quantity ?  for example..if the quantity excceds specific limit ..apply this discount on the grant total ?

Regards