I don't know the answer, but I think it would make sense to consider which offer came in first:

First case: you ask for 100 and then I bid 105 --> transaction clears at 100, the ask price

Second case: I bid 105 and then you ask for 100 --> transaction clears at 105, the bid price

This is because I implicitly think about bid offers as "I want to buy this for at most X dollars" and ask offers as "I want to sell this for at least X dollars".

I might be completely wrong though.