What did Lavan mean by "rather, than give her to someone else"?
Targum Yonasan: He was tricking Yakov by throwing him off-guard (in his plan to give him Le'ah), by implying that there was another suitor who wanted Rachel, but that he preferred him. 1
Tosfos ha'Shalem (2): It is better to give her to you, even though you do not give to me anything, than to give her to someone else, who would give to me much money for her.
See Or ha'Chayim. It is also possible that the trick Lavan was playing on Yakov was to get him to remain an extra seven years (by giving him Le'ah first), so that the well, which began flowing the moment Yakov rolled the stone off it (refer to 29:10:4:1), would continue to flow for another seven years - See Targum Yonasan on Pasuk 22). (EC)
Why did Lavan always agree to any condition that Yakov set (See also later, Pasuk 28 & 34)?
Oznayim la'Torah #2: 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 agreed with any stipulation that Yakov 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 - Le'ah 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 'Sh'va Imadi!'. 1 He merely declared that it was better to give Rachel to Yakov than to somebody else. So if he would later give him Le'ah - 'in accordance with local custom' he could not be accused of going back on his word.
See Oznayim la'Torah.
Why did Lavan, an idolater, agree to give his sister and daughters to Yitzchak and Yakov? 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.