ВАЖНО UpdateSourceTrigger=EXPLICIT
пришлось использовать ValidationRule если нужен красный бордер на 3,
просто установить цвет бесполезно - его перекрывает триггер контрола, а делать свой шаблон нах надо

public class DoubleTextBox : TextBox
{
    private BindingExpression _be = null!;
    private readonly bool _designMode;

    public DoubleTextBox()
    {
        TextChanged += OnTextChanged;
        DataContextChanged += OnDataContextChanged;
        _designMode = DesignerProperties.GetIsInDesignMode(this);
    }

    private void OnTextChanged(object sender, TextChangedEventArgs e)
    {
        if (!string.IsNullOrWhiteSpace(Text))
        {
            if (Text.EndsWith(CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator))
            {
                if (_be! is { } be)
                {
                    be.ValidateWithoutUpdate();
                }
                return;
            }

            if (!IsInputNumeric(Text, CultureInfo.CurrentCulture))
            {
                Text = string.Empty;
                return;
            }
        }

        if (!_designMode)
            _be.UpdateSource();
    }

    [MemberNotNull(nameof(_be))]
    private void OnDataContextChanged(object sender, DependencyPropertyChangedEventArgs e)
    {
        _be = GetBindingExpression(TextProperty)!;
    }


    #region Overrides of TextBoxBase

    /// <inheritdoc />
    protected override void OnTextInput(TextCompositionEventArgs e)
    {
        var culture = CultureInfo.CurrentCulture;
        if (TryHandleSpecialNonNumericCharacter(e, culture))
        {
            base.OnTextInput(e);
            return;
        }

        // Input is not a special character.
        // Cancel text input if non-numeric.
        e.Handled = !IsInputNumeric(e.Text, culture);

        base.OnTextInput(e);
    }

    private bool IsInputNumeric(string input, IFormatProvider culture) =>
        double.TryParse(input, NumberStyles.Number, culture, out _);

    private bool TryHandleSpecialNonNumericCharacter(TextCompositionEventArgs inputArgs, CultureInfo culture)
    {
        var input = inputArgs.Text;
        switch (input)
        {
            case var _ when input.Equals(culture.NumberFormat.NegativeSign,
                StringComparison.CurrentCultureIgnoreCase):
            case var _ when input.Equals(culture.NumberFormat.PositiveSign,
                StringComparison.CurrentCultureIgnoreCase):
                break;

            // Allow single decimal separator
            case var _ when input.Equals(culture.NumberFormat.NumberDecimalSeparator,
                StringComparison.CurrentCultureIgnoreCase):
                inputArgs.Handled = Text.Contains(culture.NumberFormat.NumberDecimalSeparator);
                break;
            default:
                return false;
        }

        return true;
    }

    #endregion
}

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

public class FullDoubleNumberhValidationRule : ValidationRule
{
    private readonly ValidationResult _validResult = new(true, null);
    private readonly ValidationResult _errorResult = new(false, string.Empty);

    public override ValidationResult Validate(object? value, CultureInfo cultureInfo)
    {
        if (value is string text)
        {
            if (text.EndsWith(CultureInfo.CurrentCulture.NumberFormat.NumberDecimalSeparator))
                return _errorResult;
        }
        return _validResult;

    }
}

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

 <controls:DoubleTextBox d:Text="100">
     <controls:DoubleTextBox.Text>
         <Binding
             Converter="{StaticResource SpentConverter}"
             Mode="TwoWay"
             Path="Spent"
             UpdateSourceTrigger="Explicit">
             <Binding.ValidationRules>
                 <controls:FullDoubleNumberhValidationRule />
             </Binding.ValidationRules>
         </Binding>
     </controls:DoubleTextBox.Text>
 </controls:DoubleTextBox>