tst> create table payment(id number, contr_id number /* номер контракта */, amount number);

Table created.

tst> insert into payment /* ну вот в таком порядке вводились данные */
  2  select 1 id, 1 contr_id, 1000 amount from dual union all
  3  select 2 id, 1 contr_id, 500 amount from dual union all
  4  select 3 id, 1 contr_id, -1000 amount from dual union all
  5  select rownum + 3 id, trunc(rownum/3)+2, mod(rownum, 3)*500 from dual connect by level <= 10; /* здесь какие-то другие данные */

13 rows created.

tst> create trigger paym_trg
  2  for update on payment
  3  compound trigger
  4      type amount_table is table of number;
  5      a_t amount_table;
  6      cur_sum number;
  7  before statement is
  8  begin /* это пример, поэтому мы работаем только с контрактом 1 */
  9      select amount bulk collect into a_t
 10      from payment where contr_id=1 order by id; 
 11      dbms_output.put_line('Before statement');
 12      for i in 1..a_t.count loop /* чтоб не лезть в таблицу -- сохраняем данные в локальную PL/SQL коллекцию перед началом оператора */
 13          dbms_output.put_line('--- id='||i||', a_t(i)='||a_t(i));
 14      end loop;
 15  end before statement;
 16  before each row is
 17  begin
 18      cur_sum := 0;
 19      a_t(:new.id) := a_t(:new.id) - :old.amount + :new.amount; /* и синхронно обновляем запись в локальной коллекции */
 20      dbms_output.put_line('Update row with id='||:new.id);
 21      for i in 1..a_t.count loop
 22          dbms_output.put_line('--- id='||i||', a_t(i)='||a_t(i));
 23          cur_sum := cur_sum + a_t(i); /* подсчитываем сумму для контракта при обновлении каждой строки */
 24      end loop;
 25      dbms_output.put_line('Current sum='||cur_sum);
 26      if cur_sum > 1500 then  /* вот тут нам по-идее нужно генерить ошибку и прерывать оператор */
 27          dbms_output.put_line('!!! ERROR !!!');
 28      end if;
 29  end before each row;
 30  end;
 31  /

Trigger created.

tst> set serveroutput on
tst> update payment set amount=amount*3 where contr_id=1;
Before statement
--- id=1, a_t(i)=1000
--- id=2, a_t(i)=500
--- id=3, a_t(i)=-1000
Update row with id=1
--- id=1, a_t(i)=3000
--- id=2, a_t(i)=500
--- id=3, a_t(i)=-1000
Current sum=2500
!!! ERROR !!!
Update row with id=2
--- id=1, a_t(i)=3000
--- id=2, a_t(i)=1500
--- id=3, a_t(i)=-1000
Current sum=3500
!!! ERROR !!!
Update row with id=3
--- id=1, a_t(i)=3000
--- id=2, a_t(i)=1500
--- id=3, a_t(i)=-3000
Current sum=1500

3 rows updated.

tst> rollback;

Rollback complete.

tst> create index payment_idx on payment(contr_id, id desc);

Index created.

tst> update payment set amount=amount*3 where contr_id=1;
Before statement
--- id=1, a_t(i)=1000
--- id=2, a_t(i)=500
--- id=3, a_t(i)=-1000
Update row with id=3
--- id=1, a_t(i)=1000
--- id=2, a_t(i)=500
--- id=3, a_t(i)=-3000
Current sum=-1500
Update row with id=2
--- id=1, a_t(i)=1000
--- id=2, a_t(i)=1500
--- id=3, a_t(i)=-3000
Current sum=-500
Update row with id=1
--- id=1, a_t(i)=3000
--- id=2, a_t(i)=1500
--- id=3, a_t(i)=-3000
Current sum=1500

3 rows updated.