The Vendor can issue tax invoice before you pay them. Whenever you receive the original tax invoice (only the original) you can claim purchase VAT. If tax invoice have wrong price or quality is not good then the vendor have to issue credit note for that tax invoice and Debit note or Credit note have to submit VAT to deduct from purchase VAT.
It is not matter the payment is done. It’s depend on you receive Tax invoice and Debit note or Credit note.
It is difference from withholding tax that is relate to payment date. When you pay the due, Vendor give you the receipt in case of buying goods and sometime vendor give you receipt/tax invoice in case of service.
This is the reason for different supporting document
Withholding tax – the receipt/tax invoice was required that relate to service.
VAT – The original tax invoice was required, both of service and goods