client/vendor volume discount

We have agreed on a volume discount between client and some vendors for various accounts/content type based on monthly invoice amount

For the client we invoice all related tasks across multiple projects so it is easy to manage/control - when invoicing at the end of the month the PM manually applies a discount to the total monthly amount to comply with the volume discount. 

However, from the vendor side, it is more tricky as they can select what to invoice from the vendor portal on a monthly basis. 
Let's say the volume discount kicks in at $100. They have 3 jobs to invoice this month $40/$40/$40. They could just invoice 2 jobs this month to be below the volume discount and keep pushing jobs out until a month is below the threshold. So this isn't ideal.

We then thought about creating a Rebate project and instead of having a volume discount for the amount invoiced, we would use jobs/$$ ordered for the month instead. So if in August we ordered $110, in September we would add a negative $2 PO.
The problem here though is the vendors can just choose not to invoice these negative POs.

How do you manage volume discounts in XTRF?