Payment thorugh cash ledger only ?

You should make partly payment in cash and in ledger balance

I think we are filed GSTR-9 this time you have any mistake then you Rectify ( for Example- claim excess ITC or short payment of liablity )that case you utilised the credi ledger amount but in your case is Demand then you can pay cash ledger only. its my openion But you can wait other people answer

Section or rule provision please, or any case laws please

In terms of section 49(7) read with Rule 85(3) of CGST Rules, 2017, subject to the provision of section 49, payment of every liability by a registered person as per his return shall be made by debiting the electronic credit ledger maintained as per Rule 86 or electronic cash ledger maintained as per Rule 87 and the electronic liability register shall be credited accordingly.

Refer section 49(7)