A well established firm within the building trade, are giving good discounts on some of their products which incudes the time fitting on the job.
The paperwork is legal as we have had them work for us before.
Because of the good discounts, they much prefer to have the payment in cash and not by Bankers Draft.
I'd agree - cash doesn't always go through the books, perhaps they have some casual labour who they pay in cash.
I must admit I don't like doing this, I ask contractors for a receipt for the amount (so I can have proof who did it and when, in case of later problems) and they often don't like it.