Oracle PL/SQL Trigger to update a outstanding field on one table when another table is updated with date

So I have example 2 tables:

movies:

• mid – pk varchar2
• MovieName – varchar2
• outstanding – num

rental:

• sid – num pk
• mid – fk varchar2
• rent_out – date
• rent_in – date

Need to have trigger that would decrement value in movies.outstanding when rental.rent_in is populated.

I tried to write something like this in trigger

outstanding = outstanding - 1
when mid = old.mid

but when trigger is fired it increase number instead to decrease. I used for each row and after update. Need help with this please.

So my first trigger to update rental table is:

create or replace 
  trigger rentals_rent_trg 
  after insert on rentals 
  for each row 
begin
  update Movies
  set outstanding = outstanding + 1
  where mid = :new.mid;
end;

Please I need help with 2nd trigger.
This is my 2nd trigger, it is not correct, I have tried few things but is not working:

create or replace 
  trigger rentals_return_trg 
  after update of return_date on rentals 
  for each row 
begin
update Movies
  set outstanding = outstanding - 1
  where :old.mid = :old.mid;
end;

I have tried to write 2nd trigger like this and then I am receiving ORA-04091….RENTALS is mutating, trigger/function may not see it

create or replace 
trigger rentals_return_trg 
after update of return_date on rentals 
for each row 
declare
    return_date_temp date;
begin
    select return_date
    into return_date_temp
    from rentals
    where return_date = :new.return_date
    and return_date is not null;

if return_date_temp is not null
then
      update Movies
     set outstanding = outstanding - 1
     where mid = :new.mid ;
/*mid = :old.mid;*/
end if;
end;

Since I was getting that error I have created compund trigger, however now I managed to start decreasing number however instead to be based on rows it is based on run I belive since it lowers only for one digit down if :old.outstanding = 5 then :new.outstanding = 4. I really need HELP please hahaha.

create or replace trigger rentals_return_trg 
FOR update of return_date on rentals 
COMPOUND TRIGGER
      cursor return_date_cur is
      select R.rid, R.mid, R.return_date, M.outstanding
       from rentals R JOIN MOVIES M
       ON R.mid = M.mid;
       type return_typ is table of return_date_cur%ROWTYPE
       index by binary_integer;
      return_tbl return_typ;
      INT number(4) := 0;
BEFORE STATEMENT IS
begin
for rec in return_date_cur loop
      int := int +1;
      return_tbl(int).rid := rec.rid;
      return_tbl(int).mid := rec.mid;
      return_tbl(int).return_date := rec.return_date;
       return_tbl(int).outstanding := rec.outstanding;
     end loop;
END BEFORE STATEMENT;
AFTER EACH ROW IS
    LV_RETURN_DT DATE;
    LV_OUTSTANDING_NUM NUMBER;
BEGIN
   for i IN 1..return_tbl.count loop
        if return_tbl(i).return_date = :NEW.return_date THEN
        LV_OUTSTANDING_NUM :=  return_tbl(i).outstanding - i;
       exit;
       end if;
  end loop;
  if :new.return_date is not null then
  update movies
  set outstanding = LV_OUTSTANDING_NUM;
  end if;
end after each row;
end;