Transitional input tax credit rules govern refunds, credit notes and recovery treatment on migration to the GST regime. Section 142 prescribes transitional rules for the Tripura GST transition: returns of goods sold under the existing law and returned after the appointed day may permit refund of tax paid under the existing law when returned by unregistered persons within six months; returns by registered persons are deemed supplies. Price revisions under pre existing contracts permit supplementary invoices or debit notes for increases and credit notes for decreases, with credit note effect on tax liability tied to corresponding reduction in the recipient's input tax credit. Proceedings and refund claims under the existing law are to be disposed of under that law, refunds paid in cash under that law, and amounts rejected or reversed are not admissible as input tax credit under this Act.