How much do you claim as an expense when you have used a face value voucher?

I recently purchased a £50 business item and used a £10 voucher towards the payment. How much should be claimed as the expense value? £50 (the full item value) or £40 (less the £10 voucher used)? The invoice provided states the full item value of £50 with no mention of the £10 voucher used. My business bank account statement shows £40 spent.

Hi,
It is still the £50 purchase price.
Thank you.