I think this should do what you are looking for but it is untested because I don't have ddl to work with.

create trigger TRG_CONTRIBUINTES_VERIFCA_RF_E_NIF on contribuintes

for update

as begin

update contribuintes

set nif = d.nif

from inserted i

join deleted d on d.SomeID = i.SomeID

where i.numeroposto = 'Central'

update contribuintes

set cod_rep_fiscal = d.cod_rep_fiscal

from inserted i

join deleted d on d.SomeID = i.SomeID

where i.cod_rep_fiscal not in (select cod_rep_fiscal from V_RETURN_RF)

end