First of all GST Is charged only on supply of goods or services or both. In your case there is no supply. Recovery of tax and duties of existing law i.e. excise,vat etc in GST resigme is payable as arrear of tax and paid as GST. This payment is to be made through DRC 03. please refer circular 58CGST And para 6 of circular 88 of CGST.
Insuch case Intere received can not be considered as addition in value under sec 15. As there is no value of supply.
Para 6 of circular 88 is reproduce here under for your ready reference
6. Circular No. 58/32/2018 dated 04.09.2018
This circular is revised in order to streamline the modes of recovery. Accordingly, the original and the amended relevant para of the circular are detailed hereunder.
6.1 Original Para 3.
Currently, the functionality to record this liability in the electronic liability register is not available on the common portal. Therefore, it is clarified that as an alternative method, taxpayers may reverse the wrongly availed CENVAT credit under the existing law and inadmissible transitional credit through Table 4(B)(2) of FORM GSTR-3B. The applicable interest and penalty shall apply on all such reversals which shall be paid through entry in column 9 of Table 6.1 of FORM GST-3B.
6.2 Amended Para 3.
It may be noted that all such liabilities may be discharged by the taxpayers, either voluntarily in FORM GST DRC-03or may be recovered vide order uploaded in FORM GST DRC-07, and payment against the said order shall be made in FORM GST DRC-03. It is further clarified that the alternative method of reversing the wrongly availed CENVAT credit under the existing law and inadmissible transitional credit through Table 4(B)(2) of FORM GSTR-3Bwould no longer be available to taxpayers. The applicable interest and penalty shall apply in respect of all such amounts, which shall also be paid in FORM GST DRC-03.