Oracle SQL - procedure to update another table

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SQL

Oracle

So my procedure should add a row to the table INSÄTTNING. Some keys are reference keys. When the row has been inserted it should take P_BELOPP and subtract it with SALDO which is in another table called KONTO. The new amount should be added into that column and replace the old amount. I get that SALDO must be declared error. Would it even work as i intend? Hope its pretty understandable :)

What I have tried:

create or replace procedure do_insättning(
p_radnr in insättning.radnr%type,
p_pnr in insättning.pnr%type,
p_knr in insättning.knr%type,
p_belopp in insättning.belopp%type,
p_datum in insättning.datum%type) 
as 
begin 
insert into insättning(radnr,pnr,knr) 
values(select radnr from kontoägare,select pnr from bankkund, select knr from konto);
insert into insättning(belopp,datum)
values(p_belopp,p_datum);
update konto 
set konto.saldo = konto.saldo - p_belopp
where kontoägare.radnr = p_radnr; 
dbms_output.put_line('Saldo: '||''||saldo);
end;