6 months rule applies when a registered company has claimed the Input Tax Credit arising from the purchases he made from a supplier, but has subsequently failed to pay his supplier within 6 months from the date of supply.
At the expiration of the 6 months, he has to pay back the Input Tax Credit to Customs he claimed earlier due to non-payment to his supplier.
Once he has later paid the supplier, he can then claim again the Input Tax Credit from Customs Department.
Important Notes To Claim Input Tax Credit
A company can claim Input Tax Credit from Customs Department once he receive tax invoice from the supplier, even though he has not paid the supplier!
The same goes to Supplier, the supplier will need to pay the GST to Customs Department he charge to his customer, even though he has not collected any payment from his customer.
The supplier can apply for refund from Customs Department if the customer remained unsettled with the outstanding, provided that there are proof from supplier that the supplier has been sending statements & put in efforts to get his customer pay the outstanding amount.