>If I'm running this code, the result is incorrect, because VFP cannot handle that large numbers.
>What would be a workaround?
>

>CREATE CURSOR cuTest (test c(20))
>FOR lnNr = 1 TO 7000
>	INSERT INTO cutest VALUES ("123456789012345")
>endfor
>SUM VAL(test) TO lntest
>

Does this work?

SELECT CAST(VAL(test) AS Currency) AS nBigNumber ;
    FROM cuTest ;
    INTO CURSOR c_temp

SUM nBigNunber TO lnTest

* Or, use a direct SUM() in the SQL SELECT.