VAT:administration

Sir, Illustration1 of Kaplan study text in its notes section states that:

“Relief on irrecoverable debts is given on the discounted amount. ”
I don’t understand this statement sir. Does this mean to suggest that if a trade discount was offered and the customer did not pay at all(let aside accepting the discount and paying up early) we would ask the HMRC for a reimbursement for say, only 90% of original Output VAT(if 10% trade discount) despite customer not paying up early.

Thanks in advance.