create table account
(
	id integer,
	balance numeric,
	constraint pk_account primary key (id)
);

create table transaction
(
	id serial, 
	idaccount int, 
	amount numeric, 
	constraint pk_transaction primary key (id),
	constraint fk_transaction_idaccount foreign key (idaccount) references account (id)
);

insert into account( id, balance)
select 1, 10;

begin transaction;
do
$$
declare 
	l_Balance int;
	l_IDAccount int;
	l_Amount int;
begin

l_IDAccount := 1;
l_Amount := 5;

insert into transaction( idaccount, amount)
select l_IDAccount, l_Amount;

select balance
into l_Balance
from account
where id = l_IDAccount
for no key update;

raise notice '%', l_Balance;

update account
set Balance = Balance + l_Amount
where ID = l_IDAccount;

end
$$

commit;