Unjust enrichment principle governs GST refund eligibility, requiring proof that tax incidence was not passed on for grant of refunds. Proper officers must determine and order payment or credit of refundable amounts under section 54(5), issuing acceptance or rejection within sixty days of application; refunds below one thousand rupees are not payable. The unjust enrichment principle requires claimants to prove that the tax incidence was not passed on before refunds are sanctioned, but does not apply to refunds arising from exports, accumulated ITC on exports and inverted rate structure, wrong tax paid, or tax on advances where no supply or invoice exists.