In simple terms, if Section 73 or 74 is involved, interest has to be paid. No benefit of the clause ''interest has to be paid only on cash payment of tax.' is available in that situation.
Pl. read carefully the following relevant extract of Section 50(1) of CGST Act and Rule 88 B of CGST Rules :-
Section 50(1) of CGST Act
"[Provided that the interest on tax payable in respect of supplies made during a tax period and declared in the return for the said period furnished after the due date in accordance with the provisions of section 39, except where such return is furnished after commencement of any proceedings under section 73 or section 74 in respect of the said period, shall be payable on that portion of the tax which is paid by debiting the electronic cash ledger.]"
Rule 88 B of CGST Rules
"(1) In case, where the supplies made during a tax period are declared by the registered person in the return for the said period and the said return is furnished after the due date in accordance with provisions of section 39, except where such return is furnished after commencement of any proceedings under section 73 or section 74 in respect of the said period, the interest on tax payable in respect of such supplies shall be calculated on the portion of tax which is paid by debiting the electronic cash ledger, for the period of delay in filing the said return beyond the due date, at such rate as may be notified under sub-section (1) of section 50."
The term 'Any proceeding' will encompass within its fold the investigation proceedings.