PL/SQL Insert Update Before Trigger
Here's a solution with a rational table design and reasonable reporting requirements.

SQL&gt; -- create test tables
SQL&gt; CREATE TABLE estevens.projects
  2  (
  3    project_id     NUMBER(6) NOT NULL,
  4    project_name   VARCHAR2(200) NOT NULL,
  5    project_budget NUMBER(10,2) not null,
  6    PRIMARY KEY (project_id)
  7  );

Table created.

SQL&gt; 
SQL&gt; CREATE TABLE estevens.project_expenses
  2  (
  3    expense_id        NUMBER(6) NOT NULL,
  4    project_id        NUMBER(6) NOT NULL,
  5    expense_amt       NUMBER(10,2),
  6    expense_date      DATE,
  7    CONSTRAINT project_expense_fk
  8    FOREIGN KEY (project_id)
  9    REFERENCES projects(project_id)
 10  );

Table created.

SQL&gt; --
SQL&gt; 
SQL&gt; --  populate tables
SQL&gt; insert into estevens.projects
  2           values (1,
  3               'DEMO PROJECT',
  4               1000.00
  5              )
  6  ;

1 row created.

SQL&gt; insert into estevens.projects
  2           values (2,
  3               'ANOTHER DEMO PROJECT',
  4               1000.00
  5              )
  6  ;

1 row created.

SQL&gt; insert into estevens.project_expenses
  2           values (1,
  3               1,
  4               600,
  5               to_date('2017-10-19','yyyy-mm-dd')
  6               )
  7  ;

1 row created.

SQL&gt; insert into estevens.project_expenses
  2           values (2,
  3               1,
  4               600,
  5               to_date('2017-10-19','yyyy-mm-dd')
  6               )
  7  ;

1 row created.

SQL&gt; insert into estevens.project_expenses
  2           values (3,
  3               2,
  4               600,
  5               to_date('2017-10-19','yyyy-mm-dd')
  6               )
  7  ;

1 row created.

SQL&gt; -- daily management report
SQL&gt; col project_name for a10
SQL&gt; select p.project_id,
  2         p.project_name,
  3         p.project_budget,
  4         sum(e.expense_amt) total_expense,
  5         case
  6           when sum(e.expense_amt) &gt; p.project_budget
  7           then sum(e.expense_amt) - p.project_budget
  8           else null
  9         end OVER_BUDGET
 10  from estevens.projects p
 11  join estevens.project_expenses e on p.project_id = e.project_id
 12  group by p.project_id,
 13         p.project_name,
 14         p.project_budget
 15  order by p.project_id
 16  ;

PROJECT_ID PROJECT_NA PROJECT_BUDGET TOTAL_EXPENSE OVER_BUDGET
---------- ---------- -------------- ------------- -----------
         1 DEMO PROJE           1000          1200         200
           CT

         2 ANOTHER DE           1000           600
           MO PROJECT


2 rows selected.

SQL&gt;  --
SQL&gt; drop table estevens.project_expenses;

Table dropped.

SQL&gt; drop table estevens.projects;

Table dropped.

SQL&gt; --
SQL&gt; spo off


You could also expand the idea to have an 'expense remaining' column.

Note that I have also replaced MS-style cAmElcAsE names with oracle-style names.  When in Rome . . .