You would need a helper dataset query that went something like :IF (@client=1)

begin

select @datefrom = dateadd(m, -6, getdate() )

You pass the results to the master dataset which would do something like

EXEC myProc @client @fromdate.This way you can use the procedure for ad-hoc purposes outside of the ssrs report. I would advise against dynamice SQL. Someone could add a command to drop a database at the end of a text field where you would type in the client. No need to have that risk.