WPF: Textbox and Binding to Double not able to type . on it

You are updating your property every time the value changes. When you type in a ., it is written into your viewmodel and the view is updated.

e.g. if you type in 100. it is rounded to 100, thus you won't see any dot ever.

You have some options to change this behavior:

use a deferred binding:

<TextBox Text="{Binding Path=TransactionDetails.TransactionAmount, 
                        Mode=TwoWay, 
                        UpdateSourceTrigger=PropertyChanged, 
                        Delay=250}" 
         Grid.Column="3" 
         Grid.ColumnSpan="2" 
         Grid.Row="5" 
         x:Name="TextBoxAmount" />

only change the value if it is different from the saved one (I'd recommend this for every binding):

private double _transactionAmount; 
public double TransactionAmount  
{
  get { return _transactionAmount; }    
  set
  { 
    if (_transactionAmount != value)
    {
      _transactionAmount = value; 
      Notify("TransactionAmount"); 
    }
  }

or use some kind of validation, e.g. ValidatesOnExceptions.

The best solution I got by using StringFormat like

<TextBox Text="{Binding TransactionDetails.TransactionAmount, Mode=TwoWay, 
UpdateSourceTrigger=PropertyChanged,StringFormat=N2}" Grid.Column="3" 
Grid.ColumnSpan="2" Grid.Row="5" x:Name="TextBoxAmount" />

Also we can go for custom string format as per requirements