I'd be wary of anyone identifying himself as the rightful owner of the crate and its contents. He didn't warn you of the danger?
Here's a better idea: let's work together to recover the crate and ensure that its contents can't hurt anyone. To protect ourselves, we're going to need a draught made from a few ingredients that can be found around the city.
Acquire some strange dust, get vials from an alchemy merchant, and pick some blood nettle from the fields north of the city.