Cracked Trackpad on Mac pro retina

I bought my Mac Pro Retina to Apple yesterday. I bought it at a different retailer. The problem was a hairline crack in the trackpad that I had noticed but though was a hair. A friend pointed out it was glass and a crack. I took it to Apple and the Genius inspected it. He said it was the result of some type of impact, although he tried searching for any sign of damage to the casing. There wasnt even a scratch. in fact, the place he called "the point of impact" was the recessed area directly in front of the pad. I asked "how do you impact an area like that". He said "the impact could have been anywhere". Maybe in california? I dont know and he didnt know how if this could have been the result of accidental damage, but naturally he was the "genius', and i had no real say in the matter. i did ask "so, your'e telling me that I caused this". He said "no, but I cant say you didnt". Doesnt that mean that anything is possible, including a manufactrer defect?

I was forced to sign that I'd pay for repairs on the computer, but it wasnt only the part that i was held responsible for. I was forced to commit to paying for the entire board its attached to because he told me "its al one piece".

That brings me to my biggest gripe. Why am I responsible to pay for he entire board when only one part is broken? After, I went to the retailer I bought it from, and they told me to get it back and they would try to handle it, but at that point I couldnt. I had an appt I couldnt miss.

Is a cracked trackpad that is not anyones fault my fault? Thanks!