tst> create table contract(id number primary key, cur_sum number default 0, max_sum number default 15000);

Table created.

tst> create table payment(id number primary key, c_id number references contract, ammount number);

Table created.

tst> create trigger contr_trg before insert or update on contract
  2  for each row
  3  begin if inserting then
  4           :new.cur_sum := 0;
  5        end if;
  6        if :new.cur_sum > :new.max_sum then
  7           raise_application_error(-20001, 'Cur_sum ('||:new.cur_sum||') > max_sum ('||:new.max_sum||')');
  8        end if;
  9  end;
 10  /

Trigger created.

tst> create trigger paym_trg after insert or update or delete on payment
  2  for each row
  3  begin update contract set cur_sum = cur_sum + nvl(:new.ammount, 0) - nvl(:old.ammount, 0);
  4  end;
  5  /

Trigger created.

tst> insert into contract values(1, 0, 100);

1 row created.

tst> insert into payment values(1, 1, 20);

1 row created.

tst> select * from contract;

        ID    CUR_SUM    MAX_SUM
---------- ---------- ----------
         1         20        100

tst> insert into payment values(2, 1, 90);
insert into payment values(2, 1, 90)
            *
ERROR at line 1:
ORA-20001: Cur_sum (110) > max_sum (100)
ORA-06512: at "U1.CONTR_TRG", line 5
ORA-04088: error during execution of trigger 'U1.CONTR_TRG'
ORA-06512: at "U1.PAYM_TRG", line 1
ORA-04088: error during execution of trigger 'U1.PAYM_TRG'


tst> update payment set ammount=10 where id=1;

1 row updated.

tst> insert into payment values(2, 1, 90);

1 row created.

tst> select * from contract;

        ID    CUR_SUM    MAX_SUM
---------- ---------- ----------
         1        100        100

tst> select * from payment;

        ID       C_ID    AMMOUNT
---------- ---------- ----------
         1          1         10
         2          1         90

tst>