Buy an App on the Mac App Store without having enough HD space
You can't easily interrupt the download or the check for enough space with a toggle or a setting. Much easier would be to use a second drive to add capacity. If you don't like moving files off the SSD, just install OS X onto the external drive and keep space there for overflow files and new app downloads. A 32 GB SD card or fast thumb drive would let you install a minimal OS and have 20+ GB free for app installs and other storage needs you can't accommodate on your main storage. 



The downside to installing a second OS is you have to shut down the primary and select the secondary and now you have two OS to patch. This only works if you really can't free up space on the main SSD. 

If Apple implements app gifting like it does on the iOS and iTunes stores, then you could gift a paid purchase to yourself without needing the download to commence when the sale is made. 

Similarly, setting up a volume purchase account would also let you "purchase" apps from the web interface and not from the App Store interface. It's unlikely any of this is easier than freeing up space on the OS you run or bringing a new OS with enough space to your Mac.