I'm sorry your good intentions, focused just on getting an explanation for your kid, turned into a gossipy mess. It WILL blow over.
Meanwhile, you say you called the "organizer" but later say you called the "company." So... did you talk with a parent who organized this for your school, or did you call the business that organizes this? If it's the latter, you can't help that they called the school though I don't know why an assistant principal would get his or her knickers in a twist over this whole issue.
I would tell the assistant principal (and/or the parent coordinator for this fundraiser if there is one):
"Just to follow up on the fundraiser: I called to ask about that prize issue not because I was after a prize for Sally but simply because she was upset and I was just seeking a way to explain things to her. I figured getting more information would help me talk with her. But now I'm hearing from other parents that my call, which wasn't even TO the school, is being talked about around school. That's really disappointing to me that this is now common knowledge because it was a question intended just to get information, not to put on pressure to give my child something. I hope that if you hear parents around school discussing it, you will let them know the subject's closed as far as the school is concerned -- because the subject is certainly closed for our family."
That doesn't directly accuse the assistant principal or parent-organizer of being the one to start gossiping (though it seems to me that this person is the one who DID start that ball rolling) but it does put her or him on notice that you know you are the topic of gossip and you expect it to get quashed. Might not work if the person or people is/are jerky, or just super-sensitive, but it's worth a try.
In most schools there would be a PTA committee or PTA parent heading up these things and that would have been my first stop in a case like this, not the company that provides the goods and prizes. In your school, maybe the fundraisers are the special baby of this assistant principal....? That is a good clue to stay clear of fundraisers. Our new school's PTA does NO fundraising; they are very up front about saying that they operate on donations alone bccause they know that no one likes fundraising.