If you're using VFP9, then transform your funciton into derived table, e.g.

select R.*, CAST(NVL(Expended,0) as I) as expended from rta_elements R LEFT JOIN 
(   select  sum(amount) as Expended, Sponsor_ID ;
from tip ;
inner join ledger on ledger.tip_id = tip.tip_id and source="RTA" ;
group by Sponsor_ID) MyAmount ON R.ElementNum = val(substr(MyAmount.sponsor_id,5,2))