Input tax credit entitlement hinges on documentary proof, receipt, tax payment, and timely filing; unpaid supplier amounts reverse credit. Every registered person may claim input tax credit for business use where they possess a tax invoice or prescribed document from a registered supplier, have received the goods or services, the tax has been actually paid to the Government (in cash or via usable credit), and the return under section 39 has been filed. Credit for instalment receipts accrues only on the last instalment, credit is reversed if the recipient fails to pay the supplier within the prescribed period, credit is barred where depreciation on the tax component was claimed, and claims are time limited by the September return or the annual return deadline.