а так ли нужен курсор ?
может через lateral одним запросом ?


begin

declare local temporary table tb(
ID integer,
DAT date,
ID_material integer,
KOL decimal (17,5),
TP integer
);

declare local temporary table tb_tmp(
ID integer,
DAT date,
ID_material integer,
KOL decimal (17,5),
TP integer
);


update tb set tp=1 where tb.id in (
select C.id
from tb A join tb_tmp B on A.id=B.id,
lateral(select first C.id from tb C where C.dat<A.dat and C.id_material=A.id_material order by C.dat desc,C.id) C
where B.kol<0 and C.id is not null);

commit;



end