Recovery (against cylinders not returned) is not additional consideration OR even a consideration for 'gas' supplied. Hence, debit-note u/s 34 cannot be raised.
Debit-note u/s 34 (3) can be raised only where one or more tax invoices have been issued for supply of any goods or services or both and the taxable value or tax charged in that tax invoice is found to be less than the taxable value or tax payable in respect of such supply.
Recovery (against cylinders not returned) - in given scenario - can NOT lead to charge that the taxable value or tax charged in that tax invoice (i.e. original invoice) is less than the taxable value or tax payable in respect of such supply of original supply i.e. "GAS".
Hence, debit-note u/s 34 cannot be raised, in my view, against recovery for cylinders not returned in given scenario.
W.r.t. issues that may arise in raising debit-notes u/s 34, I have already explained them in my earlier post.
These are ex facie views of mine, the same should not be construed as professional advice / suggestion. & I respect contrary views.