You are getting the null value because you are not getting a match in the table.
Since,DateTime.Now.ToString() gives the output in the format "dd/MM/yyyy hr:min:sec". Which includes the time(hr:min:sec). So to check the date of the leave. You only need to send the (dd/MM/yyyy) part of today's date.
Which you can achieve by using this code
DateTime dtm = DateTime.Now;
string date = dtm.ToString("dd/MM/yyyy");
Pass this 'date' as parameter.
Next in the database, you would need to check the leave date as
select CONVERT(VARCHAR(10),leaveDate, 103) as RequestedDate from Table_Name
It will give you the value in the format(dd/MM/yyyy).
Hope this solves your issue.
Thanks!!