I need to receive an invoice pre service in order to submit it to my finance department for payment. Is this possible?

I work in the public sector.  All purchases need to be presented to my finance department as an invoice for payment.  The only other option is for me to pay via a personal credit card and then be reimbursed.  Because this is a service purchase, I am not asking for product before payment, I am just wanting to receive an invoice so I can get you payment for a new account!