If the part payment is made to the supplier, the reversal will be proportionate to the amount not paid to the supplier.In other words, the recipient (buyer/receiver of goods or services) is entitled to the ITC to the extent of payment made against supply.
Proviso to Section 16(2) talks of _________fails to pay__________the amount. It does not talk of whole payment. If the recipient fails to pay 50%, ITC is to be reversed on 50% of payment not made. In other words, credit on the amount of 50% is allowed. Emphasis is supplied on 'failure to pay'. i.e. ITC is to be disallowed only on the portion of payment not made.
Punishment is for failure and not for the period for which the receiver has complied with.
Proviso to Section 16(2) says
Provided further that where a recipient fails to pay to the supplier of goods or services or both, other than the supplies on which tax is payable on reverse charge basis, the amount towards the value of supply along with tax payable thereon within a period of one hundred and eighty days from the date of issue of invoice by the supplier, an amount equal to the input tax credit availed by the recipient shall be added to his output tax liability, along with interest thereon, in such manner as may be prescribed.
It is applicable for both goods and services.
A period of 180 days is to be calculated in view of time of supply. Time of supply is different/separate for goods and services.