Why did Avraham need to say "if they will not give the woman to you, you will be absolved from this curse"?
Kidushin, 61b: We learn from here that a stipulation that is not like that of Bnei Gad and Bnei Reuven, which was doubled - is invalid. 1
See Torah Temimah (8, footnote 15) who discusses why the Gemara learned from here, and not from Pasuk 8.
Why does it say both "Tinakeh me'Alasi" and "v'Hayisa Naki me'Alasi"?
Malbim: If you go to my family, you are exempt from the Shevu'ah to go to my family. If they do not let her go with you, you are exempt from the Shevu'ah not to take a Kena'anis.
Avraham said (verse 8) "if she woman will not go with you, you will be absolved." Why did Eliezer say "if they will not give her to you, you will be absolved"?
Ha'Emek Davar: Avraham said to be Mekadesh Stam, and she will need to come, to avoid being Agunah. Eliezer saw that it is better to stipulate now that she must come to Yitzchak, lest Yitzchak need to wait (until she realizes that he will not come, and she must go to avoid Igun her entire life). 1
If Eliezer thought that this is better, why did he not tell Avraham immediately? Perhaps only now, after he saw the Hashgachah, he sensed that her family will agree to the stipulation. (PF)