Since the above suggestion didn't work, so I had done it like below.
In .ascx, defined the field inside webdatagrid as under:
<ig:UnboundField Header-Text="Cash Received" Key="Cash Received"></ig:UnboundField>
Inside InitializeRow event defined it like below:
protected void WebDataGrid1_InitializeRow(object sender, Infragistics.Web.UI.GridControls.RowEventArgs e)
{
if(e.Row.DataItem !=null)
{
Client cln=e.Row.DataItem as Client;
if(cln.CashRec < 0)
{
String strCR = cln.CashRec.ToString("f2"); //Two places of decimal
e.Row.Items[4].Value= string.Format("-${0}", strCR.Substring(1, strCR.Length -1)); //Added minus sign & Dollar symbol
}
else
e.Row.Items[4].Value= string.Format("${0}",cln.CashRec); //Added Dollar symbol
}
}
