What was the extent of Lavan's efforts to gain at Yakov's expense?
Ramban: Following the initial agreement, that Yakov would receive the spotted and blotched sheep and goats and the reddish-brown sheep, he began changing the conditions annually, when he saw the extent of Yakov's successes.
Why did the Mal'ah say "Im Koh Yomar" in the future?
Ramban: Because Hashem formed the lambs that would be formed each year in advance of the new conditions that Lavan set - after the sheep were already pregnant.
How could Lavan say "your wage will be the dotted ones"? They already agreed that Yakov gets the dotted and blotched!
Moshav Zekenim: Yakov stipulated that Lavan may choose each year to give to him dotted, blotched or Akudim.
Why does it say "Im Koh Yomar" in the future?
Ramban, Ohr ha'Chayim: Lavan changed the conditions after the animals were already pregnant - but to no avail. Hashem took into account what Lavan will say, and formed the seed inside the mothers in Yakov's favor. Ha'Emek Davar