Nested sp_executesql not working with output variable - sql-server

Question

I am trying to call a stored procedure (with output variable) using sp_executesql but within another stored procedure. I wrote the following, but still not able to get trhough what that error means
This would be called from webservice code:
exec sp1 'obj1',#params
Here obj and params are of nvarchar(max)
Definition of sp1 is :
Alter procedure [dbo].[sp1 ]
#procname nvarchar(max),
#params nvarchar(max)
as
declare #temp varchar(15)
if #procname = 'obj1'
begin
set #params = #params + ',#Newval varchar(15) output'
EXEC sp_executesql #sp2,#params,#Newval=#temp OUTPUT
end
Definition of sp2:
Here I am returning #Newval
Error I am getting :
Msg 102, Level 15, State 1, Line 1
Incorrect syntax near ','.
Also in 2 in place of exec statement , I have tried following:
EXEC sp_executesql #sp2, #params, #temp OUTPUT;
Results in the same error.
set #sql='sp2,' + ' #params ' + ',#params,#temp OUTPUT'
EXEC sp_executesql (#sql)
Also results in the same error.
I need this dynamic selection of stored procedures in sp1 and params is a nvarchar(max) string of parameters and their values, some of them are varchar and are embedded in ''value'' format but this string is fine as I am able to call the underlying sp2 with this.
Additional info, it it helps.
EXEC sp_executesql #sp2,#params,#Newval=#temp OUTPUT
in this #params is combination of keys and vlaue pairs for the final sp. something like :
'#key1="a",#key2="b"'
and so on, I can not predefined the #params but it is dynamic and it is working
fine when I run it with
exec (#sql)
Format while whole of the name, params are embedded in the #sql

Answer 1

If #params='' or NULL then your , before #Newval is irrelevant. I suggest you to check:
IF NULLIF(#params,'') IS NULL or #params IS NULL
SET #params = '#Newval varchar(15) output'
ELSE
SET #params = #params + ',#Newval varchar(15) output'
You are passing #sp2 maybe you need this:
ALTER PROCEDURE [dbo].[sp1]
#procname nvarchar(max),
#params nvarchar(max)
AS
DECLARE #temp varchar(15)
IF #procname = 'obj1'
BEGIN
SET #params = #params + ',#Newval varchar(15) output'
EXEC sp_executesql N'EXEC sp2 #someparam1, #someparam2, #Newval varchar(15) OUTPUT', #params, #someparam1 = 1, #someparam2 = 2, #Newval=#temp OUTPUT
END
EDIT
Working example:
USE [AdventureWorks]
GO
DECLARE #procname nvarchar(max) = 'EXEC [dbo].[uspGetWhereUsedProductID] #StartProductID, #CheckDate',
#params nvarchar(max) = '#StartProductID int, #CheckDate date'
EXEC sp_executesql #procname, #params, #StartProductID = 1, #CheckDate = '2015-10-17'
GO