I would suggest you might need some guidance for HR or payroll personnel, but not a policy, which I take to mean something for employees to read. Has anything prompted this? Something particular in your sector, maybe?

Hi Penny

This is a matter that's strictly regulated by employment law anyhow, so unless your employers particularly feel they have to insist that all written deduction consents are signed in blood and /or in the presence of the local Mayor etc then it's just a waste of paper / everyones's time

(crossed with Nick's similar but less flippant response)

Hi Penny

I would expect to see this covered in your contracts of employment, not a policy document. If you intend to rely on the policy to stop money from people's pay lawfully, you will have to make it contractual. If it isn't contractual, it won't enable you to deduct a penny more than you can at the moment. I, too, would kick this one into the long grass.