Overriding Invoice Tax Amount

Hi all,

Is there any way, using the API, to replicate clicking the Tax arrow button and overriding the Tax Amount of an Invoice? This brings up the tax Information window and you can manually enter the Tax Amount.

I have a client who wants to post their invoices as they are into MYOB with custom tax amounts, and no matter what I do, MYOB will always recalculate the amounts. In the example attached, I posted an Invoice with the Tax Amount change to 26.00 as seen in the "TotalTax" value. But this and the TotalAmount values seem to always be ignored.