Matching of input tax credit: discrepancies lead to addition to recipient's output tax liability with interest and possible refund. Section 42 creates a matching process for inward supplies against suppliers' outward supplies and IGST on imports; matched claims are accepted. Discrepancies or undeclared outward supplies are communicated and, if not rectified, added to the recipient's output tax liability; duplication of claims results in addition of excess credit to output tax. The recipient may reverse additions if the supplier later declares the invoice within the prescribed time; interest is payable on added amounts until rectification, and accepted reductions lead to an interest refund to the recipient's electronic cash ledger subject to limits.