Outward sale quarterly return

you have a few options to consider:

1. File an amendment: You can file an amended GSTR-1 for December, correcting the mistakes and adjusting the sales figures. However, this option is only available if you haven't yet filed the GSTR-3B for the relevant period.

 2. File a rectification request: Reach out to the GST department and submit a rectification request explaining the mistake and providing the correct information. They may consider your request and allow rectification.

3. Wait for the GST audit: If the above options aren't feasible, you can wait for the GST audit. During the audit, you can highlight the mistake and provide the correct information. The auditor may consider it and make the necessary adjustments.

 4. Consult a tax professional: It's always a good idea to consult a tax professional or chartered accountant to explore the best course of action and ensure compliance with GST regulations.

Keep in mind that rectifying mistakes in GST filings can be complex and time-sensitive.