>(a) You were talking abstractly about "donations", not about Microsoft, (b) no, not for tax purposes. If you give someone a million dollars and then are reimbursed, say, a quarter of that by someone else, it's simply dishonest to publicly claim that you donated a million dollars, as (ba) your assets have only decreased by 750000 USD due to the donation, and (bb) because you are taking credit for the quarter million that that other person/entity paid for.

I'm not sure you understand how tax deductions work.

Donating to charity allows you to claim a portion of that donation as a deductible, this can vary depending circumstances but in most cases you have a limit for the deductibles for gross income and the amount it self is usually not deducted in full regardless of limits when it comes to non-cash property (this is also gets more complicated because you have carrybacks, other losses, expenses etc.).

When Microsoft donates X amount of what ever to some one no one pays MSFT that money back, they might get some tax relief for it that's all this doesn't mean that you as an individual lose money in fact direct donation in many cases is more efficient than indirect funding through taxation.

>Except that you choose to simply ignore one giant transaction in this market that happened only once they selectively reduced the price to zero (or rather, might happen, if people don't still consider that to be too expensive). If they were giving away a hundred dollars worth of services, sure, I'd be willing to accept that as a reasonable method of valuating the donation, but if it's in the same order of magnitude as their total yearly revenue in this area, not so much.

It doesn't affect the price of the service in any way regardless of what portion of the global XAAS market or MSFT's own XAAS business it is. This simply allows for organizations that would not afford access to these resources at this scale before to have the opportunity to do so.
It's also spread across 3 years and includes all MSFT cloud services so Office 365, Skype for Business/Lync, OneDrive etc. which makes is about 1/8th of their currently yearly revenue from cloud products which means overall it's about 1/24th of it's yearly cloud related revenue over the duration of the program.

>because of proprietary interfaces--the different providers certainly are in competition to one another, but one's product is not necessarily a direct substitute for the other's.

This holds true even if you build your infrastructure and platform in house it's not like MSFT can donate universal credits

>A better measure in this case would be how much you could get others to buy it from you for. If your rock massively increases the supply of rocks, that is going to considerably drop the price.

I am a non-profit or a school I don't sell rocks but I do need one, not everyone operates a tradable commodity exchange...
You don't value the price of donated food at the local homeless soup kitchen by how much you could sell it to some one on the street you value it based on how much you will have to pay to make it on your own.
And the people who actually receive the food tend to value it based on how hungry they are....