Step to reproduce

1: Select european format (1.000.000,00) for numbers in setting page
2: Set amount with dot as comma (on the numeric keypad, the comma is a dot)
3: save the invoice

Screenshots

Actually

Expected

Note: All values are hand filled before, when darkens become == click on save and when lightens it's the saved values returned by server.

Possible solution

Change standardize_amount function in helpers/number_helper.php with that

function standardize_amount($amount)
{
    if ($amount && ! is_numeric($amount))
    {
        $CI = & get_instance();
        $thousands_separator = $CI->mdl_settings->setting('thousands_separator');
        $decimal_point = $CI->mdl_settings->setting('decimal_point');

        if ($thousands_separator == '.' && ! substr_count($amount, ',') && substr_count($amount, '.') > 1)
        {
            $amount[ strrpos($amount, '.') ] = ','; // Replace last position of dot to comma
        }

        $amount = strtr($amount, [$thousands_separator => '', $decimal_point => '.']);
    }
    return $amount;
}

Note

This fix improve the function

• Applied only on not numeric
• str_replace to strtr
• Replace last position of dot to comma if don't have a comma
• And work with all number format chosen by user

This not solve all problems of typos like 1.00.0 > 100,00 but just trust on human intellect after all.