Bind Multibinding Textbox in WPF MVVM - wpf

Question

I have 3 TextBoxes bind with my class(Transaction) properties like this
<TextBox Text="{Binding Path=Transaction.Bills100,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" Name="bills100" Grid.Column="2" Grid.Row="1" Margin="7"></TextBox>
<TextBox Text="{Binding Path=Transaction.Bill50,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" Name="bills50" Grid.Column="2" Grid.Row="2" Margin="7"></TextBox>
<TextBox Text="{Binding Path=Transaction.Bill20,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" Name="bills20" Grid.Column="2" Grid.Row="3" Margin="7"></TextBox>
Also I have another TextBox where I have done multibinding and done addition of the first three Textboxes like
<TextBox Grid.Column="2" IsReadOnly="True" Grid.Row="7" Grid.ColumnSpan="2" Margin="7" Name="TotalBills">
<TextBox.Text>
<MultiBinding Converter="{ikriv:MathConverter}" ConverterParameter="x+y+z" Mode="TwoWay">
<Binding Path="Text" ElementName="bills100" />
<Binding Path="Text" ElementName="bills50" />
<Binding Path="Text" ElementName="bills20" />
</MultiBinding>
</TextBox.Text>
</TextBox>
I want to bind this multibinding textbox with my class(Transaction) with property as Transaction.Total like my first three textboxes but it shows error
Property text is set more than once

Answer 1

Actually we cannot get the value of a two-way binding from one property and then set the value of another property.
Finally I came with a solution like this
In my Class Transaction
private double _totalBills;
public double TotalBills
{
get { return _totalBills; }
set { _totalBills= value; Notify("TotalBills"); }
}
In XAML(Instead of Multibinding)
<TextBox Text="{Binding Path=Transaction.TotalBills,Mode=TwoWay,UpdateSourceTrigger=PropertyChanged}" Grid.Column="2" IsReadOnly="True" Grid.Row="7" Grid.ColumnSpan="2" Margin="7" Name="TotalBills"/>
My ViewModel
public class MainViewModel: INotifyPropertyChanged
{
private Transaction _transactionDetails;
public MainViewModel()
{
Transaction= new Transaction();
_transactionDetails.PropertyChanged += _transactionDetails_PropertyChanged;
}
private void _transactionDetails_PropertyChanged(object sender,PropertyChangedEventArgs e)
{
switch (e.PropertyName)
{
case "TotalBills":
_calculate(); //My method for calculation
break;
}
}
}