Pass data from SSRS report back to SSRS reports main Stored Procedure

In my SSRS report, I can tell if a user selected all the choices in the drop down parameter by doing:

=IIF(COUNTROWS("SC_procRptLstBusinessUnit").Equals(Parameters!BusinessUnit.Count), "All ", (Join(Parameters!BusinessUnit.Label, ", ")))

Then in my report, I would show the word 'ALL' somewhere in the report instead of listing out all the individual choices.

Now, my question is:

How can I pass back to my Stored procedure, via a parameter, the letter 'A',  so I know that the user chose all the choices from the drop down parameter ?

I tried passing back a parameter (@BusinessALL) to the SP but this code was not allowed in my parameter expression:

=IIF(COUNTROWS("SC_procRptLstBusinessUnit").Equals(Parameters!BusinessUnit.Count), "A", Nothing)


A aggregate or Lookup function are not allowed in a query parameter function.