I am facing strange issue here..

I have added a global charge named SURCHARGE.
I have used added it everywhere wherever required as below:

def _compute_amount(self):
        """
        Compute the total amounts and GST amount of the Invoice.
        """
        self.amount_untaxed = sum(line.price_subtotal for line in self.invoice_line_ids)
        self.amount_before_surcharge = self.amount_untaxed
        self.fuel_surcharge_amt = self.amount_untaxed * self.fuel_surcharge/100

        self.amount_untaxed += self.fuel_surcharge_amt

def get_taxes_values(self):
        tax_grouped = {}
        for line in self.invoice_line_ids:
            price_bfc = line.price_unit * (1 - (line.discount or 0.0) / 100.0) + line.invoice_ins_charge
            price_unit = price_bfc + (price_bfc* self.fuel_surcharge/100)
            taxes = line.invoice_line_tax_ids.compute_all(price_unit, self.currency_id, line.quantity, line.product_id, self.partner_id)['taxes']
            for tax in taxes:

Now, the Invoice Total amount due didnt show the price including the new surcharge, so added to residual as well.

def _compute_residual(self):
        residual = 0.0
        residual_company_signed = 0.0
        sign = self.type in ['in_refund', 'out_refund'] and -1 or 1
        for line in self.sudo().move_id.line_ids:
            if line.account_id.internal_type in ('receivable', 'payable'):
                residual_company_signed += line.amount_residual
                if line.currency_id == self.currency_id:
                    residual += line.amount_residual_currency if line.currency_id else line.amount_residual
                else:
                    from_currency = (line.currency_id and line.currency_id.with_context(date=line.date)) or line.company_id.currency_id.with_context(date=line.date)
                    residual += from_currency.compute(line.amount_residual, self.currency_id)
        
        
        residual+=self.fuel_surcharge_amt
        residual_company_signed += self.fuel_surcharge_amt

        self.residual_company_signed = abs(residual_company_signed) * sign
        self.residual_signed = abs(residual) * sign
        self.residual = abs(residual)
        digits_rounding_precision = self.currency_id.rounding
        if float_is_zero(self.residual, precision_rounding=digits_rounding_precision):
            self.reconciled = True
        else:
            self.reconciled = False

Now, after this the Invoice shows correct total everywhere untill I register payment.
Suppose my Invoice has total due as below

Product Price 100.00 Fuel Surcharge(Amt)  10.00

Untaxed Amount 110.00₹ Tax 19.80₹ Total 129.80₹
Amount Due 129.80₹

-----------------------------------

Now, when I register payment, It tells me total amount due as 129.80, I am able to make complete payment..

But after registering payment, the new amount structure is as below:

Product Price 100.00 Fuel Surcharge(Amt) 10.00 Untaxed Amount 110.00₹ Tax 19.80₹ Total 129.80₹

The surchrge added is not taken in payment and it shows as amount due and credit amount.
Please help... what I am missing here.. ??