1) Your constant character array is too small to hold the value of the null terminated initial string.
2) As well, your declaration of the function is incorrect.
extern void verkoop(char *productid, char aantal*);
should be
extern void verkoop(char *productid, char *aantal);
(HINT: Look at '*' for aantal);
3) You call the function with invalid parameters. The function call should have been
verkoop(productid, aantal);
Could you share more info about the compiler that you're using ?