"Found Money": excuse, or frugality perk?

Sometimes in life we get money that we don't expect, or save money that we had accounted on spending. How should we treat this?

Here are a few examples. I opened a recent bill to find that I had been overcharged $39 the last time I was billed. As a result, this money is applied to my current bill, which as it happens means it covers it all (and lowers the next bill). That is $39 I had basically spent, that I'm now getting back (sure I was never supposed to have spent it, but in the initial bill the overcharge did not stand out to me--it was what I had reasonably expected to pay). I also planned on buying about three pairs of pants in August. In a recent excursion through storage areas of my home, I found three pairs of pants (one brand new, two still in good shape) that now fit rather nicely. I had guessed on spending around $30, and now I don't have to.

Now, there are also many cases in life when we have to pay for things we didn't expect to, or have to pay more than we accounted for. Perhaps "found money" just balances things out. Perhaps that's how we should treat it, and we should save any "found money."

But...

When you're living a frugal lifestyle, that "found money" can feel a lot like "free money." Based on the examples I provided above, I feel like I should now have about $69 to spend on whatever I want. After all, it's money I either didn't plan on having, or planned on spending, that I now get to keep. I don't need it--it had already been budgeted away. Why shouldn't I now spend that money on something extra? $69 is like 12 or 13 veggie subs at Subway. It's almost a hobby box of Topps 2008 football cards. Considering I'm plus $69, couldn't I eat guilt-free at Subway for 12 meals? Couldn't I buy myself that box of football cards without any remorse?

Am I making excuses to spend money I don't need to spend, or am I finding a way to live life both frugally and enjoyably?