It may be as simple as Moridin was allowed to enter and leave freely. This assumes that he had made some agreement previously where he would recieve a specified item, i.e. Dark Haired Aes Sedai, woman who could channel however worded. Upon inspection of Moirane, he simply stated she was not the item, thus the bargain was not completed and he left until such time as his item could be delivered.
After reading my above rambling, I also realize the 'Finns may have tried to cheat him by offering Moraine. He simply refused to accept the packagae, then 'Finns delivered the promised goods (Lanfear.) The motive for the 'Finns could be found in Moiranes post rescue words about them feeding on her ability to channel. Lanfear would have been naturally stronger in the Power thus they would have liked to keep her.
Anyways just some thoughts on this post.