Hello Syncfusion,
Here is MyCode
<syncfusion:SfDataGrid x:Name="dataGrid" ColumnSizer="Auto" AllowGroupExpandCollapse="True">
<syncfusion:SfDataGrid.Columns x:TypeArguments="syncfusion:Columns">
<syncfusion:GridTextColumn MappingName="MainGroupItemName" IsHidden="True" />
<syncfusion:GridTextColumn MappingName="Total" IsHidden="True"/>
<syncfusion:GridTextColumn HeaderText="Περιγραφή" MappingName="Name" HeaderCellTextSize="14" CellTextSize="14" />
<syncfusion:GridTextColumn HeaderText="Ποσ." MappingName="Quantity" Width="80" Format="c2" HeaderCellTextSize="14" CellTextSize="14" />
<syncfusion:GridTextColumn HeaderText="Τιμή" MappingName="Price" Width="80" Format="c2" HeaderCellTextSize="14" CellTextSize="14" />
<syncfusion:GridTextColumn HeaderText="Σύνολο" MappingName="DiscountTotal" Width="80" Format="c2" HeaderCellTextSize="14" CellTextSize="14" />
</syncfusion:SfDataGrid.Columns>
<syncfusion:SfDataGrid.GroupColumnDescriptions>
<syncfusion:GroupColumnDescription ColumnName="MainGroupItemName"/>
</syncfusion:SfDataGrid.GroupColumnDescriptions>
<syncfusion:SfDataGrid.CaptionSummaryRow>
<syncfusion:GridGroupSummaryRow ShowSummaryInRow="False">
<syncfusion:GridGroupSummaryRow.SummaryColumns>
<syncfusion:GridSummaryColumn Name="DiscountTotal" Format="{}{Sum:c2}" MappingName="DiscountTotal" SummaryType="DoubleAggregate" />
</syncfusion:GridGroupSummaryRow.SummaryColumns>
</syncfusion:GridGroupSummaryRow>
</syncfusion:SfDataGrid.CaptionSummaryRow>
<syncfusion:SfDataGrid.TableSummaryRows>
<syncfusion:GridTableSummaryRow >
<syncfusion:GridTableSummaryRow.SummaryColumns >
<syncfusion:GridSummaryColumn Name="DiscountTotal" Format="{}{Sum:c2}" MappingName="DiscountTotal" SummaryType="DoubleAggregate" />
</syncfusion:GridTableSummaryRow.SummaryColumns>
</syncfusion:GridTableSummaryRow>
</syncfusion:SfDataGrid.TableSummaryRows>
</syncfusion:SfDataGrid>
Thank you Suga, customtemplate is what i was looking for