Hi,

Try with the following code

class PurchaseOrder(models.Model):

    _inherit = 'purchase.order'

    discount_amount = fields.Monetary(string='Discount Amount')

    tax_amount = fields.Monetary(string='Manual Tax Amount')

    @api.depends('amount_untaxed', 'tax_amount', 'discount_amount')

    def _compute_amount(self):

        for order in self:

            order.amount_tax = order.tax_amount  # override tax if needed

            order.amount_total = (

                order.amount_untaxed + order.tax_amount - order.discount_amount

            )

Odoo already computes amount_untaxed and amount_tax, so you might override amount_tax with your manual value (order.tax_amount), and then adjust the total.

-Don't try to write order.tax_totals = {...} — it's a computed UI field.

-Instead, override _compute_amount() to update amount_total based on your logic.

-Ensure amount_total is updated via this method, as Odoo uses it for invoice creation, vendor bills, and reports.

Hope it helps

Hii,

Override the tax_totals property on the purchase.order model.

Compute and return the adjusted values in the getter.

Example Fix

from odoo import models, api

class PurchaseOrder(models.Model):

    _inherit = 'purchase.order'

    @property

    def tax_totals(self):

        self.ensure_one()

        order_lines = self.order_line.filtered(lambda x: not x.display_type)

        # Step 1: Get base tax totals

        base_totals = self.env['account.tax']._prepare_tax_totals(

            [line._convert_to_tax_base_line_dict() for line in order_lines],

            self.currency_id or self.company_id.currency_id,

        )

        # Step 2: Adjust with manual values

        computed_tax = base_totals.get('amount_tax', 0.0)

        tax_adjustment = self.tax_amount - computed_tax

        tax_totals = base_totals['amount_total'] + tax_adjustment - self.discount_amount

        # Step 3: Return modified totals

        return {

            **base_totals,

            'amount_tax': self.tax_amount,

            'amount_total': tax_totals,

            'formatted_amount_total': self.currency_id.format(tax_totals),

        }

tax_totals is primarily used in portal views, PDF reports, or frontend rendering.

You don’t store this field — it’s used at read-time (e.g., in templates or JSON responses).

If you want to store a modified total (e.g., for backend use), create a new fields.Monetary field like custom_total_amount and compute it separately.

i hope it is usefull