How to reflect local corporate tax deductions from exports as a reduction in sales

Our company is a manufacturing exporter that handles everything from production and delivery to direct sales. In the past, when exporting directly to certain countries, we recognized the sales amount based on the Export Declaration value, which included the collectible revenue plus other items (local corporate tax, commissions, etc.). Upon reviewing the actual contract terms, we found that the country in question did not have a double taxation avoidance agreement prior to a certain point in time, our company has no permanent establishment (PE) in that country, and the client's contract included a clause to deduct a portion of the corporate tax rate from the recognized import amount. We have determined that the local corporate tax amount is not a variable consideration, sales return, or allowance, but rather a deduction from the gross sales previously recognized; therefore, we are reflecting it as a reduction in sales this time. I am wondering whether this deduction should be reported as a difference in the general tax base during this month's VAT filing, or if it should be reported by separately indicating the negative amount in a footnote on the Export Declaration issued at the time of the initial shipment.