Purchased digital copy, only to have it go on sale via Games with Gold next day. Is it ...

You would have to contact support and speak to billing, but generally they don't give partial refunds. 1-800-4my-xbox if you're in the US/Canada or
support.xbox.com/contact-us

Just food for thought. The physical Media Disc is being rendered useless starting Xbox One Gen, so if this is the case and Game/s are also Day One purchase from Digital this begs an interesting question.

If I purchase a Game either via Disc (which is nothing but a dust magnet now) or via Day One Digital (going to be my preferred method) and a Copy of my Game is accessible from any Console via Cloud Storage, do I have to actually keep even the Digital Copy of
the Disc on my HDD? I mean after all, it's available to me from any Console and is ready to go? Why waste space on my HDD with an Installed Game? Is there something more to all of this?

Hi, you are right for some of what you said. Yes you should be able to download the games if they are backward compatible, these will be shown in the ready to install list in my games and apps, this refers to digital copies only. Any disc copies, you have
to have the disc to insert, and then download the update to the game. You have to keep the disc in the drive to play the game. As for a discount to purchase the game, no that is not true, unless the game is available in the deals with gold or a sale is on.

Now a days it's an issue with even disc games. I have the same issues with disc copies I have. The only reason I have so many digital is because I saved a lot of money on sales and such that I wouldn't of on disc copies.

by making games digital only sales would take a huge hit, no company would risk it. I could see them releasing digital copies earlier than disc but never going away from disc sales.