From what I remember, Littlefinger didn't know about Ramsay's craziness which is really poor writing. Just endorsing what people said in this thread.
Anyway, I doubt Sansa would forgive him after what Ramsay did to her. I hope it's just D&D screwing their plot and GRRM has a better idea.
Well, Roose knew about Ramsay's craziness and he still didn't expect him to lose Sansa. Littlefinger had to assume she would at least be alive and there by the time he came back, that's not a crazy assumption even with the rumors that must go around about Ramsay.
Sansa will either forgive him or better yet, she'll pretend to forgive him to get his army.