substantive procedures for sales discount

I don’t think you should be trying to plug what you might think are gaps in your knowledge at the last minute. You may well see something in the exam that you haven’t thought about before and you are going to have to think of something yourself.

Rather than substantive procedures on lots of small amounts, it’s likely that the client will have controls in place to ensure that customers are only given the benefit of prompt-payment (i.e. settlement) discounts that they are entitled to – and therefore that the auditor would test controls on allowing discounts.