Answer by Squirrel 1

    declare @DMA1Var int            
    declare @PrevDMA1 decimal(29,12)            
    declare @DMA1 decimal(29,12)            
-- changed the select statement            
    select @sql = N'select @PrevDMA1 = DMA1 from ' + quotename(@TableName) + N' where id = @PrevID '             
-- added sp_executesql here            
    EXEC sp_executesql @sql, N'@PrevID int, @PrevDMA1 decimal(29,12) OUTPUT', @PrevID, @PrevDMA1 OUTPUT            
    select @sql = N'select @DMA1 = cast(((RawLastPrice - @PrevDMA1) * (2 / (@DMA1Var + 1)) + @PrevDMA1) as decimal(29,12)) from ' + quotename(@TableName)            
    select @params = N'@DMA1 decimal(29,12) OUTPUT, @NewID int, @DMA1Var int, @PrevDMA1 decimal(29,12) OUTPUT, @PrevID int'            
    EXEC sp_executesql @sql, @params, @DMA1 = @DMA1 OUTPUT, @NewID = @NewID, @DMA1Var = @DMA1Var, @PrevDMA1 = @PrevDMA1 OUTPUT,@PrevID = @PreviousID