Va'eira

In Berachos 9a,b Rav Yannai says that the reason it says in Shemos 11,2 "daber NA... viyishalu ish maies ra'aihu... Klei chesef uklei zahav" (speak please... and let each man borrow from his friend... silver and gold vessels) is because na means please and Hashem was telling Moshe "Please. Go tell Klal Yisrael please borrow gold and silver vessels because that way Avraham won't be able to say 'you kept(Beraishis 15,13) 'they will serve them and oppress them' but not 'afterwards they will leave with tremendous wealth' '!" Why is the problem that Avraham is going to complain, the problem should be that Hashem has to do it since he promised to?