With some kind of award for not using all your money, it's hard to justify it for the people spending it... and "rewards for not spending all the money" are also really easy to cut out of next year's budget...
Certain patterns just naturally emerge with budgeting at scale, and if you've got some brilliant idea about how to fix it I can assure you that with just some minimum business acumen you can become a very rich person.
So, where did that $20 go ? To some other team, right ?
What about the reverse scenario? If I cannot do all my work with $80, I ask for more, say $100. Then my budget for next year becomes 100.
I don't see a problem with this. Budgets are fluid and change from year to year. Because I saved the company $20 in year 6, I will be rewarded with $20 in year 7 if I ask for it.
Also, it is not limited to gorvernment: a Director in a Fortune 100 company that I'm working for right now is buying 40 16" MacBook Pros for a team of .Net developers, because:
a) he had spare $100k in his budget.
b) it is easier to get VPN access approved for external contractors if they are using Macs
Nothing, absolutely nothing that is used in the company is related to the Mac. In fact, a lot of the tools we use would work better on Windows, and many people use Windows. The Macs for direct employees are used as perks.
[0] not sure if this is still true, but certainly was in 80s-early 00s.