Pre-Payment Invoices for Grant-Funded Non-Profit Organizations

Our 501c3 non-profit org can prove state tax-exempt status. Need to purchase $8,500 worth of laptops but fed funding entity requires a pre-payment invoice for payment. Once they receive it, we will be able to draw down funds into our bank account to cover online purchase payment with our org credit card.

How do we go about securing a pre-payment invoice from Apple? The suggestions herein all refer to after-payment circumstances. Thank you in advance.