Update statement not working within Cursor & While block

Question

I am trying to execute an Update statement on a table. This statement is placed within Cursor and While block. I have checked in debugger and the values are coming into the statements and variable, still the update is not putting values into the table fields. Please advise what am I doing wrong here.

ALTER PROCEDURE SP_PO1  @P1 int

AS
BEGIN

    SET NOCOUNT ON;

    DECLARE @DOC AS INT;
    DECLARE @CASH AS FLOAT;
    DECLARE @TENTYPE AS VARCHAR(100);
    DECLARE @UDF AS VARCHAR(100);
    DECLARE @COUNTER AS INT;
    DECLARE @SQL AS VARCHAR(500);

    SELECT @DOC=DOCTYPE FROM InvNum WHERE AutoIndex = @P1;
    IF @DOC = 6
        BEGIN
            SET @COUNTER = 1;
            DECLARE Cur_Tender CURSOR FOR
                SELECT Tender.TenderNo FROM Tender;

            OPEN CUR_TENDER;
            FETCH NEXT FROM CUR_TENDER INTO @TENTYPE;

            WHILE @@FETCH_STATUS = 0
            BEGIN

            SELECT    @CASH = ISNULL(_btblPOSTenderTx.fTxAmount,0) FROM _btblPOSTenderTx INNER JOIN Tender ON _btblPOSTenderTx.iTenderID = Tender.IdTender INNER JOIN _btblPOSXZTable ON _btblPOSTenderTx.iPOSXZTableID = _btblPOSXZTable.IDPOSXZTable WHERE (_btblPOSXZTable.iTillTxType = 7) and (_btblPOSXZTable.IDPOSXZTable = (select Max(IDPOSXZTable) from [dbo].[_btblPOSXZTable])) AND (TenderNo = @TENTYPE);

                SET @UDF = 'ufIDPOSInvTENDER' + CONVERT(VARCHAR(2),@COUNTER);

                UPDATE InvNum SET @UDF=@CASH WHERE AutoIndex = @P1;

                SET @COUNTER = @COUNTER + 1;

                FETCH NEXT FROM CUR_TENDER INTO @TENTYPE;
            END
        END
    CLOSE CUR_TENDER
    DEALLOCATE CUR_TENDER
END
GO

Answer 1

You update variable

 UPDATE InvNum SET @UDF=@CASH WHERE AutoIndex = @P1;

update table column

 UPDATE InvNum SET <column>=@CASH WHERE AutoIndex = @P1;

if you want a dynamic column name - use dynamic sql

 EXEC('UPDATE InvNum SET ' + @UDF + '=' + CAST(@CASH as VARCHAR(50) + ' WHERE AutoIndex = ' + CAST(@P1 as VARCHAR(5) ' );