How can I manually override the tax amount on an A/R down payment invoice?

Hi SAP Business One community

To comply with Australian GST legislation, when we issue the first A/R down payment invoice relating to a contract/sale we have to recognise and remit GST on 100% of the total sales order value at that time. Ie the rules state that the full amount of GST must be remitted to the ATO a) when we receive the first instalment payment, or b) when we invoice the customer for the goods (whichever comes first). So for example, we issue an a/r downpayment invoice for 15% of the order value, but we still have to remit 10% of the total order value for GST purposes at that time. I can't find a way to manually override the tax amount in the A/R down payment invoice and then eventually the A/R invoice itself to ensure the correct amount is showing for tax in the document and our tax reports. I need it to show correctly on our tax declaration / tax reconciliation reports for BAS reporting. Any help would be much appreciated. Or referrals to a company you have used in Australia in the past with a similar issue that has been resolved. We use SAP Business One 9.3.

Thanks