The syntax you use for your left join is incorrect. You can use

select ...;
from apdetl;
left join appay on apdetl.apdetlid=appay.apdetlid ;
where apdetl.apmasid=apmas.apmasid

Or

select ...;
from apdetl;
where apdetl.apmasid=apmas.apmasid and apdetl.apdetlid=appay.apdetlid

HTH

---

>select apdetl.ivcdate, apdetl.ivcnum, apdetl.duedate, apdetl.amount, sum(appay.amount), apdetl.amount-sum(appay.amount) ;
>from apdetl left join appay ;
>where apdetl.apmasid=apmas.apmasid and apdetl.apdetlid=appay.apdetlid ;
>into cursor curLstApDetl ;
>order by apdetl.ivcdate