Unjust enrichment: Do the arrangements apply to part reimbursements?
Yes. Such cases occur when it is agreed that the business paid some of the tax being refunded rather than receive it from its customers. For example, a business may submit a refund claim for £50,000, and agree that £30,000 was passed on to customers, but ask to be allowed to keep £20,000 because it paid that amount from what would otherwise have been part of its profits. In such circumstances only £30,000 would be subject to the arrangements. The £20,000 claim would be considered separately by HMRC on its merits.
However, you will need to agree with the business what percentage of the refund will be passed back to its customers before the undertaking is signed.