> SELECT d.amount INTO amt FROM ARBOR.BILL_INVOICE b,
> ARBOR.BILL_INVOICE_DETAIL d WHERE
> b.index_bill_ref = d.index_bill_ref AND b.index_bill_ref_resets =
> d.index_bill_ref_resets;
is where your problem is. Try replacing it with

SELECT amount INTO amt
FROM BILL_INVOICE_DETAIL
WHERE index_bill_ref = :newRow.index_bill_ref
AND index_bill_ref_resets = :newRow.index_bill_ref_resets;

and the trigger should work (but not necessarily correctly - I am not
sure that detail record is already there when the trigger on master
table fires, thus the statement may throw NO_DATA_FOUND.)
Besides the fact that you are selecting from the same table the trigger
is defined on (which is generally prohibited - that's why you get
ORA-4091), your original statement will select more than one row
and will fail because single row is expected.

--
Vladimir Zakharychev (bob@dpsp-yes.com) [url]http://www.dpsp-yes.com[/url]
Dynamic PSP(tm) - the first true RAD toolkit for Oracle-based internet applications.
All opinions are mine and do not necessarily go in line with those of my employer.