What did Lavan mean by, "It is better to give her to you, than to give her to someone else"?
Targum Yonasan: He told him this deceitfully. 1
Tosfos ha'Shalem (2): It is better to give her to you, even though you are not giving me anything, than to give her to someone else, who would give to me much money for her.
Ohr ha'Chayim: He was scheming to get Yaakov to work additional years. He would explain, you will work for me for seven years, and for this I agree to marry her to you (for additional work) rather than to give her for free to someone else. Yaakov thought that he meant 'there is no one better than you to whom to give her!'
Malbim: He was scheming to get Yaakov to work additional years. He said, there is no one better than you to whom to give her. You need not work for me at all. Stay with me, and I will support you, so you will marry her - without any conditions. Yaakov guarded his flock as Chesed, so when Lavan will say, "we do not give the younger before the older...," Yaakov cannot say, "we agreed...."
Ha'amek Davar (to 29:19,20): Yaakov wanted to marry her immediately, and work afterwards, like he did after marrying Leah. Lavan said, I will give her only after she is proper to give to another man. Yaakov thought that it is because she is a minor, or weak. Yaakov accepted this. "Va'Ya'avod Yisrael b'Ishah uv'Ishah Shamar" (Hoshe'a 12:13) - he worked for her before the Nisu'in, and waited until the time for the Nisu'in.
He did not explain what the deceit was. Below (29:22), Targum Yonasan says that [seven years later,] Lavan asked Bnei Charan how to trick Yaakov into remaining in Charan another seven years! Perhaps he implied that there was another suitor who wanted Rachel, but that Lavan preferred Yaakov. (EC)
Why did Lavan always agree to any condition that Yaakov set (see also 30:28, 34)?
Oznayim la'Torah #1: An astute businessman who abides by his word will seek the best price and even haggle to obtain it, because once he has clinched the deal, he knows that he cannot retract. Not so Lavan, who was ready to agree with any stipulation that Yaakov put forward in order to give him a good feeling, thereby encouraging him to work honestly and give of his best - in the knowledge that he would later go back on his word and change the conditions as he saw fit - Leah instead of Rachel (excuse in hand); changing the conditions once, twice... ten times!
Oznayim la'Torah #2: Lavan did not say 'Yes!', nor did he say 'Avod Imadi!', but rather 'Shevah Imadi!' 1 He merely declared that it was better to give Rachel to Yaakov than to somebody else. So if he would later give him Leah - 'in accordance with local custom' he could not be accused of going back on his word.
Why did Lavan, an idolater, agree to give his sister and daughters to Yitzchak and Yaakov? He knew that they will not let them serve idolatry!
Tosfos ha'Shalem (1, citing Imrei No'am): R. Yehudah ha'Chasid asked his father, who answered that the idolaters believed in Hashem, just they served idolatry to know future events.
Why did Lavan add, "Shevah Imadi"?
Ohr ha'Chayim #1: Stay close, and do not distance yourself (Lavan wanted to oversee his work).
Ohr ha'Chayim #2: He stipulated that Yaakov stay permanently, just like Yisro stipulated with Moshe. 1 He intended to use this if needed, but Hashem told him not to - "Hishamer Lecha Pen Tedaber Im Yaakov" (31:24). Yaakov did not understand his intent; he thought that he said so amidst love.
Ha'amek Davar: Engage in work until the time to marry her. Shevah also implies waiting.
Ohr ha'Chayim: Yisro made Moshe swear not to leave without his permission, to include even if Tziporah will agree.