Using 45W Power Adapter with MacBook Pro Retina 15

You can safely use the 85W adapter on any Mac, it will only draw as much power as it needs. The 45W adapter from the MacBook Air may just about manage to power your MacBook Pro but it won't charge it. It will not damage your Mac, but you may notice the battery may still deplete when using it.

Here is the information, straight from Apple:

Apple only produces an adapter to connect the original Magsafe to Macs with Magsafe 2. It doesn't provide one to go the other way around. At the moment I'm unaware of any third party product that does this (and given the lack of third party Magsafe accessories in general, I wouldn't expect one any time soon).