The DataGrid cell in edit mode is not accepted. (Dot) for deimal value in WPF MVVM

I have a DataGrid and whose column is bound to the decimal properties of the ViewModel.

The decimal value is displayed when the value is added from the ViewModel, but when I edit the same cell and delete the contents and then add, the cell does not accept .Period. How can I edit a cell, then.

<DataGrid Name="dgSales" Height="300" Margin="0,3,0,0" IsSynchronizedWithCurrentItem="True" SelectedItem="{Binding SelectedItem,Converter={StaticResource ignoreNewItemPlaceHolderConverter}}" ItemsSource="{Binding StockList,UpdateSourceTrigger=PropertyChanged}" ToolTip="{Binding RelativeSource={RelativeSource Self},Path=(Validation.Errors)[0].ErrorContent}" GotKeyboardFocus="dgSales_GotKeyboardFocus" CurrentCellChanged="dgSales_CurrentCellChanged">
    <DataGrid.Columns>
        <DataGridTextColumn Header="S No." Width="SizeToCells" MinWidth="60" Binding="{Binding SNo}" IsReadOnly="True" />
        <DataGridTemplateColumn Header="Stock Name" Width="280">
            <DataGridTemplateColumn.CellTemplate>
                <DataTemplate>
                <TextBlock Text="{Binding StockName}"/>
                </DataTemplate>
            </DataGridTemplateColumn.CellTemplate>
            <DataGridTemplateColumn.CellEditingTemplate>
                <DataTemplate>
                    <ComboBox Width="280" Name="cmbStock" ItemsSource="{Binding Path=Stocks}" FocusManager.FocusedElement="{Binding RelativeSource={RelativeSource Self}}" ></ComboBox>
                </DataTemplate>
            </DataGridTemplateColumn.CellEditingTemplate>
        </DataGridTemplateColumn>
        <DataGridTextColumn Header="Unit Price" Width="SizeToCells" MinWidth="80" Binding="{Binding UnitPrice,UpdateSourceTrigger=PropertyChanged}" />
        <DataGridTextColumn Header="Unit" Width="SizeToCells" MinWidth="80" Binding="{Binding Unit}" IsReadOnly="True"/>
    <DataGridTextColumn Header="Quantity" Width="SizeToCells" MinWidth="80" Binding="{Binding Quantity,UpdateSourceTrigger=PropertyChanged}" />
        <DataGridTextColumn Header="Discount" Width="SizeToCells" MinWidth="80" Binding="{Binding Discount,UpdateSourceTrigger=PropertyChanged}"/>
        <DataGridTextColumn Header="Amount" Width="SizeToCells" MinWidth="100" Binding="{Binding Amount,StringFormat=\{0:n2\}}" IsReadOnly="True" />
    </DataGrid.Columns>
</DataGrid>