Dear Shri Alkesh Jani Ji,
First, please be clarify that I missed the fact that Post No. 8 was from you. I somehow thought the same is from original querist (as I had expounded my thought-process in post at serial No. 7). And hence, my reply at serial No. 9 was actually addressed to him. My apologies to you and Shri Ashwin Jivanlal Ji, the original querist for this misunderstanding!
Now, please let me try to answer questions raised by you, to the best of my understanding of law in the context of subject discussion & my earlier posts.(I find that some of your questions are open-ended and cannot be answered - at-least by me - without context behind those questions raised).
1. can a contract be unlawful, or any clause is made, so as it may defeat the provisions of any law for time being in force?
My views: Section 16 - or for that matter, any provisions of GST law - does not bar any supplier & recipient to enter into a contract of supply between them where mutually agreed terms of payment are beyond 180 days from date of invoice. Having such mutually agreed terms of payment does not defeat subject proviso under discussion here.
2. can a contract be made where one party is unable to comply with any provisions of law time being in force?
My views: I have not understood the question in the context of discussion about second proviso u/s 16 (2) (d).
3. As per GST Provision, if consideration is not paid within the limit set can be termed as “failure to pay”?
My views: There is no time limit set for payment of consideration under GST law and hence, there can not be “failure to pay” as per GST Provision/s.
4. interpretation of a proviso can be done independently?
My views: I have not understood the question in the context of discussion about second proviso u/s 16 (2) (d) read with main provisions of Section 16 (2) or Section 16 (1) for that matter.
I also wish to draw attention of wordings used for proviso under Rule 4 (7) of Cenvat Credit Rules, 2004 which reads as follows:
"Provided further that in case the payment of the value of input service and the service tax paid or payable as indicated in the invoice, bill or, as the case may be, challan referred to in rule 9 is not made within three months of the date of the invoice, bill or, as the case may be, challan, the manufacturer or the service provider who has taken credit on such input service, shall pay an amount equal to the CENVAT credit availed on such input service, except an amount equal to the CENVAT credit of the tax that is paid by the manufacturer or the service provider as recipient of service, and in case the said payment is made, the manufacturer or output service provider, as the case may be, shall be entitled to take the credit of the amount equivalent to the CENVAT credit paid earlier subject to the other provisions of these rules:"
This further support, to my mind, that there is marked difference between '.... where a recipient fails to pay to the supplier ..... ' (the wordings used in second proviso u/s 16 (2) (d)) and '....... where a recipient does not pay to the supplier ......' (the wordings which are not used in second proviso u/s 16 (2) (d)). And both type of these wordings cannot be read as equal / same.
These are ex facie views of mine and the same should not be construed as professional advice / suggestion. And I fully respect contrary views.