If you need to issue a sales document for goods or services you have not supplied yet, you can issue a 'pro-forma' invoice or a similar document to offer goods or services to customers.
A pro-forma invoice sometime called an “estimate” is not a VAT invoice, thus you should clearly identify them with the words 'This is not a VAT invoice'.
If your potential customers accept the goods or services you are offering them and if you actually supply them, then you will need to issue a VAT invoice within the appropriate time limit as noted in the VAT Act, section 35 “Time of Supply.” If you have been issued with a pro-forma invoice by your supplier, who is also registered for VAT you cannot use that to claim back VAT on the purchase. You must obtain a VAT invoice from that supplier.
Only VAT-registered businesses can issue valid VAT invoices. You cannot reclaim VAT on any goods or services that you buy from a business that is not VAT registered.