Thank you for notifying. This issue has now been fixed and the update will be available with the next release of TMS WEB Core.
Please note that there is no Total property as the total amount is calculated automatically. As a workaround you can leave the Item.Tax at 0, this won't affect the total as the Item.Price should be set to the item's price including tax.
Thanks. Currently it works correctly if I set the Item.Tax to 0 and the Payment.Tax to the total tax due. However the Item.Price then needs to be the net value.