Supplier and recepient is not related party hence sch I of CGST Act is not applicable. And Provisio to rule 37 is apliapplic only for sch I cases. Provisio of rule 37 is reproduce here under Provided that the value of supplies made without consideration as specified in Schedule I of the said Act shall be deemed to have been paid for the purposes of the second proviso to sub-section (2) of section 16:
In your case invoice is made and you design transaction as supply with consideration but afterwards recepient fails to pay. In that case sec 16(2) Provisio 2 is applicable
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 aperiod 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:
Provided also that the recipient shall be entitled to avail of the credit of input tax on payment made by him of the amount towards the value of supply of goods or services or both along with tax payable thereon.
So in your case recepient is required to reverse ITC if he doesn't pay to recepient.
If supplier actually supply goods without consideration the supplier is not eligible for ITC on his input as per sec 17(5)(h) goods lost, stolen, destroyed, written off or disposed of by way of gift or free samples;