SQL Stored Procedure Query to generate Invoice No

SP to generate InvoiceNo.

CREATE PROCEDURE [dbo].[SPGenerateInvoiceNo]
 -- Add the parameters for the stored procedure here
 @InvoiceDate datetime,
 @InvoiceNo varchar(10)='' output
AS
BEGIN
 -- SET NOCOUNT ON added to prevent extra result sets from
 -- interfering with SELECT statements.
 SET NOCOUNT ON;

 DECLARE @IncrementNo varchar(6);
    DECLARE @FinancialYear varchar(4);
    DECLARE @LastInvoiceNo varchar(50);
    DECLARE @NextInvoiceNo varchar(50);
 DECLARE @Alphabate VARCHAR(1);
   
   SET @FinancialYear = (SELECT
   CASE WHEN MONTH(@InvoiceDate)>=4 THEN
         SUBSTRING(convert(VARCHAR,YEAR(@InvoiceDate)),3,2)+SUBSTRING(convert(VARCHAR,YEAR(@InvoiceDate)+1),3,2)
   ELSE
   SUBSTRING(convert(VARCHAR,YEAR(@InvoiceDate)-1),3,2)+SUBSTRING(convert(VARCHAR,YEAR(@InvoiceDate)),3,2)
   END);
     
    IF NOT EXISTS(SELECT * FROM [InvoiceSystem].[dbo].[Tbl_Invoice] WHERE InvoiceDate > @InvoiceDate)
  BEGIN
  IF EXISTS(select TOP(1)InvoiceNo from [InvoiceSystem].[dbo].[Tbl_Invoice] order BY InvoiceNo desc)
   BEGIN
   SET @LastInvoiceNo =(select TOP(1)InvoiceNo from [InvoiceSystem].[dbo].[Tbl_Invoice] order BY InvoiceNo desc)
   END
  ELSE
   BEGIN
   SET @LastInvoiceNo = '00000000'
   END
  SET @IncrementNo = (SELECT SUBSTRING(@LastInvoiceNo,5,4));
  SELECT @IncrementNo = RIGHT('0000'+ CONVERT(VARCHAR,convert(INT,@IncrementNo)+1), 4);
  END
    ELSE
  BEGIN
   SET @NextInvoiceNo = (SELECT TOP(1)InvoiceNo FROM [InvoiceSystem].[dbo].[Tbl_Invoice] WHERE InvoiceDate> @InvoiceDate ORDER BY InvoiceNo);
   print(@NextInvoiceNo)
   Declare @len varchar(5);
   SET @len = (select len(@NextInvoiceNo));
   IF (@len <= 8)
   BEGIN
    SET @LastInvoiceNo = (SELECT TOP(1)InvoiceNo FROM [InvoiceSystem].[dbo].[Tbl_Invoice] WHERE InvoiceDate<= @InvoiceDate ORDER BY InvoiceNo DESC);
  
    SET @len = (select len(@LastInvoiceNo));
    IF (@len > 8)
     BEGIN 
     SET @Alphabate = (SELECT char(ascii(SUBSTRING(@LastInvoiceNo,10,1)) +1)); 
     SET @IncrementNo = (SELECT SUBSTRING(@LastInvoiceNo,5,4)+'.'+@Alphabate);
     END
    ELSE
     BEGIN
     IF (@len is null)
      BEGIN
      SET @IncrementNo = '0';
      END
     ELSE
      BEGIN
      SET @IncrementNo = (SELECT SUBSTRING(@LastInvoiceNo,5,4)+'.A');
      END
   
     END
   END
   ELSE
   BEGIN
    SET @IncrementNo=0
   END
  END
 IF (@IncrementNo <> '0')
  BEGIN
   SET @InvoiceNo = (SELECT @FinancialYear + @IncrementNo);
   SELECT @InvoiceNo AS 'InvoiceNo';
  END
 ELSE
  BEGIN
   SELECT '-99' AS 'InvoiceNo';
  END
END