VAT on a rebate cashback - Agent or principal?

How do businesses like Quidco structure the VAT on commission they receive?

Fantasy example:

• Landscaper offers £20 to/for new customer or referral.
• I find customer and offer them £10 rebate/cashback.
• Landscaper pays me the £20 and I owe £10 to the customer as it was never my income and owed to customer from the start.

Can this be structured from the start so that I only pay VAT on my £10 not the whole £20 as £10 is client money passing -through?

Perhaps an agreement with the source/landscaper that my fee is separate to the cashback?