Is there a way to purchase incompatible app from Mac App Store I haven't upgraded to Yosemite on my Mac yet and still using Mavericks. I wanted to purchase an app during its promotion period but the app min version is 10.10 so Mac App Store complains saying that the app cannot be installed and does not even allow me to purchase. Is there a way where I can just purchase even though I might not have to download and install it. I searched and could not find any way (iTunes does not support). 

While I might upgrade shortly, it might take time since I want to take proper backups and then upgrade during which the promotion of the app might be over.
Unfortunately, the only way I've found the App Store to cooperate with that was to create a user account on a compatible system from which to log into the App Store and download the now-compatible app. For example, on my Lion system with a 32-bit EFI, I was unable to download Mountain Lion or newer to install on my daughter's MBA. I had to log into her Air in my account to download the OS upgrade for her.

Not convenient.