I can understand why this question may arise. On some cases you’re actually paying and on some cases you decrease outstanding balances for an example.

Either way, sales bonuses you pay to your customers, they’re always, always decreasing your revenue. On the other hand bonuses you get from your suppliers decrease your expenses. It’s always about decreasing the item you got a bonus for or paid a bonus. 

You could also think they’re your expenses since it’s not your income effectively. You’re encountering an expense. But if you think about, it’s connected with sales revenues – volumes, amounts or whatever, but it’s connected with sales revenue, paid based on something in sales revenue. It should thus be recognized within sales revenue to make the total amount to reflect the actual amount you got from your sales. For an example, if you give discount if your client pays before the due date – an expense on its own or decrease in sales? Did you get the full amount of an amount less the discount? Maybe this answers your question.

If it’s something you pay after the customer has made a payment, it’s still connected with the sales revenue and you should reflect you effectively got less sales revenue.