Saving result from sp_executesql into a variable and using dynamic column name - getting error "Error converting data type varchar to numeric"

Question

Im getting an error when running a procedure that includes this code.
I need to select from a dynamic column name and save the result in a variable, but seem to be having trouble with the values being fed to sp_executesql
DECLARE @retval AS DECIMAL(12,2)
DECLARE @MonthVal VARCHAR(20), @SpreadKeyVal INT
DECLARE @sqlcmd AS NVARCHAR(150)
DECLARE @paramdef NVARCHAR(150)
SET @MonthVal = 'Month' + CAST(@MonthNumber AS VARCHAR(2) );
SET @SpreadKeyVal = @SpreadKey; --CAST(@SpreadKey AS VARCHAR(10) );
SET @sqlcmd = N' SELECT @retvalout = @MonthVal FROM dbo.CourseSpread WHERE CourseSpreadId = @SpreadKeyVal';
SET @paramdef = N'@MonthVal VARCHAR(20), @SpreadKeyVal INT, @retvalout DECIMAL(12,2) OUTPUT'
--default
SET @retval = 0.0;
EXECUTE sys.sp_executesql @sqlcmd,@paramdef, @MonthVal = 'Month4',@SpreadKeyVal = 1, @retvalout = @retval OUTPUT;
SELECT @retval
DECLARE @return_value DECIMAL(12,2)
EXEC @return_value = [dbo].[GetSpreadValueByMonthNumber]
@SpreadKey = 1,
@MonthNumber = 4
SELECT 'Return Value' = @return_value
Msg 8114, Level 16, State 5, Line 1
Error converting data type varchar to numeric.

Answer 1

Please follow basic Netiquette and post the DDL we need to answer this. Follow industry and ANSI/ISO standards in your data. You should follow ISO-11179 rules for naming data elements. You should follow ISO-8601 rules for displaying temporal data. We need
to know the data types, keys and constraints on the table. Avoid dialect in favor of ANSI/ISO Standard SQL. And you need to read and download the PDF for: 
https://www.simple-talk.com/books/sql-books/119-sql-code-smells/
>> I need to select from a dynamic column name and save the result in a variable, but seem to be having trouble with the values being fed to sp_executesql <<
This is so very, very wrong! A column is an attribute of an entity. The idea that you are so screwed up that you have no idea if you want
the shoe size, the phone number or something else at run time of this entity. 
In Software Engineering we have a principle called cohesion that says a model should do one and only one task, have one and only one entry point, and one and only one exit point. 
Hey, on a scale from 1 to 10, what color is your favorite letter of the alphabet? Yes, your mindset is that level of sillyity and absurdity. 
Do you know that SQL is a declarative language? This family of languages does not use local variables! 
Now think about “month_val” and what it means. A month is a temporal unit of measurement, so this is as silly as saying “liter_val” in your code. Why did you use “sp_” on a procedure? It has special meaning in T-SQL.  
Think about how silly this is: 
 SET @month_val = 'Month' + CAST(@month_nbr AS VARCHAR(2));
We do not do display formatting in a query. This is a violation of at the tiered architecture principle. We have a presentation layer. But more than that, the INTERVAL temporal data type is a {year-month} and never just a month. This is fundamental. 
We need to see the DDL so we can re-write this mess. Want to fix it or not?
--CELKO-- Books in Celko Series for Morgan-Kaufmann Publishing: Analytics and OLAP in SQL / Data and Databases: Concepts in Practice Data / Measurements and Standards in SQL SQL for Smarties / SQL Programming Style / SQL Puzzles and Answers / Thinking
in Sets / Trees and Hierarchies in SQL