Use a Trigger which formats the value if the TextBox isn't selected
<Style TargetType="{x:Type TextBox}">
<Setter Property="Text" Value="{Binding SomeValue, StringFormat=N2}" />
<Style.Triggers>
<Trigger Property="IsKeyboardFocusWithin" Value="True">
<Setter Property="Text" Value="{Binding SomeValue}" />
</Trigger>
</Style.Triggers>
</Style>
You can also use a Converter for formatting if you can't easily format with StringFormat
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