Was ownership of the money transferred to her at the moment that I put the money down and I told her to pick it up?

I live in an apartment with 3 other people. I owed roommate A some money and told her I would pay back some of it (all of what was due immediately and some of what was due but could be paid back at a later date). I took amount X out of my bank account and used amount Y ($20 less than X) to pay her. I left the money in a hidden but accessible location in the kitchen area and texted A to get the money from the location. The next day A texted me and told me the money was not there. Assuming she was telling the truth that the money was not there (I have zero reason to doubt her veracity), how do I go about dealing with the fact that money was stolen? She says she will not be breathing on my back to get Y back but I want to know how the situation stands legally. Was it her fault for not going back to the apartment until late the next day to pick the money up?