Problem with function & trigger

This does not work. The error is ERROR: pg_atoi: error in "12.00": can't
parse ".00"

CREATE FUNCTION "togliscar" () RETURNS opaque AS 'BEGIN
UPDATE magazzino SET quantita = (SELECT quantita FROM magazzino WHERE
descrizione = NEW.descrizione )
- NEW.quantita WHERE descrizione = NEW.descrizione;
RETURN NEW;
END;
' LANGUAGE 'plpgsql';

CREATE TRIGGER "togliscarico" BEFORE INSERT ON "scarico" FOR EACH ROW
EXECUTE PROCEDURE
"togliscar" ();

This works.

CREATE FUNCTION "delscar" () RETURNS opaque AS 'BEGIN
UPDATE magazzino SET quantita = (SELECT quantita FROM magazzino WHERE
descrizione = OLD.descrizione )
+ OLD.quantita WHERE descrizione = OLD.descrizione;
RETURN OLD;
END;
' LANGUAGE 'plpgsql';

CREATE TRIGGER "delscarico" BEFORE DELETE ON "scarico" FOR EACH ROW
EXECUTE PROCEDURE
"delscar" ();

The field quantita is numeric(10,2).

Both functions works if quantita is integer.

Any hint?

Bye!