WPF Converter JSON string to multiple textboxes

I've already made a workaround for this problem because of time constraints at work, although I still want to ask for leaing purposes.

So I had this issue where I was making an editor screen for some record data, and in this record was a field called 'Quantity'. However, when designed, it was made a quantity placeholder, but it meant different things. So to explain, it is a SkuReference table, that has a 'Type' that defines if it's a 'Quantity per Pack', 'Roll Length', or 'CBC'. Well, for 'Quantity per Pack' and 'Roll Length', a simple number works, however for the 'CBC' (meaning, Coers/Borders/Centers) the data is stored as a JSON string object:

{ 'Coers': 10, 'Borders': 20, 'Centers': 30 }

Now on the WPF screen, if the data is identified as a 'CBC', I route the data to three textboxes, all bound to the 'Quantity' property of the parent object and using a converter and parameters to identify each one and I put the appropriate value into each textbox. Works fine.

The problem I have is when trying to work the ConvertBack part of the converter. I realized that I do not have reference to the original string property that I can edit and supply the new value to, or access to the other textboxes to just rebuild a new string to retu. I was trying to come up with a resolution maybe using MultiBinding in my head, but could not completely come through with an answer.

Is this even possible? BTW I ended up just creating new properties that were split up and when the parent object was set parsed and passed around data. However, for future reference it would seem cleaner to me to just use the original data and a converter without the extra work.

Below is other code for reference:

XAML, UpsertSkuReference.Quantity is the JSON string above

        <StackPanel Grid.Column="5">
            <TextBlock Text="CBC" />
            <StackPanel Orientation="Horizontal">
                <TextBox Width="30" Text="{Binding UpsertSkuReference.Quantity, ConverterParameter=co, Converter={StaticResource CBCToIndividualConverter}}" IsEnabled="{Binding CBCIsChecked}" />
                <TextBox Width="30" Margin="5,0,0,0" Text="{Binding UpsertSkuReference.Quantity, ConverterParameter=b, Converter={StaticResource CBCToIndividualConverter}}" IsEnabled="{Binding CBCIsChecked}" />
                <TextBox Width="30" Margin="5,0,0,0" Text="{Binding UpsertSkuReference.Quantity, ConverterParameter=ce, Converter={StaticResource CBCToIndividualConverter}}" IsEnabled="{Binding CBCIsChecked}" />
            </StackPanel>
        </StackPanel>

Converter

public class CBCToIndividualConverter : IValueConverter
{
    JavaScriptSerializer json = new JavaScriptSerializer();

    public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
    {
        //--value = CBC JSON object string
        //--parameter = [co]: Coers, [b]: Borders, [ce]: Centers
        if (value != null)
        {
            if (parameter == null) { throw new Exception("CBCToIndividualConverter: parameter caot be null"); }
            if (new string[] { "co", "b", "ce" }.Contains(parameter.ToString().ToLower()) == false)
            { throw new Exception("CBCToIndividualConverter: parameter must be 'co' for Coers, 'b' for Borders, or 'ce' for Centers"); }

            CoerBorderCenterModel cbc = json.Deserialize<CoerBorderCenterModel>(value.ToString());

            switch (parameter.ToString().ToLower())
            {
                case "co": { retu cbc.Coers; }
                case "b": { retu cbc.Borders; }
                case "ce": { retu cbc.Centers; }
                default: { retu null; }
            }
        }
        else { retu null; }
    }

    public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
    {
        if (value != null)
        {
            //--value = number for parameter type
            //--parameter = [co]: Coers, [b]: Borders, [ce]: Centers

            //--?? Uh Oh
        }
        retu null;
    }
}