free or not free? that is the question

Today,

Mum found $20 lying in the grass next to our parked car.

Nothing had been prepared for dinner yet, so we said "Haha! Let's get KFC then!" And we used that $20 to buy ourselves dinner.

So.

My question is-

Was the dinner really a free dinner?

I mean, you can take it one of two ways. We found the money, so now it's ours, and we chose to spend it when really, we could have saved it to be better off by $20. Or alternatively, we've just spent somebody else's money, so the only difference between the moments of pre-discovery and post-discovery of the $20 note was the dinner that we acquired. There was no financial gain nor loss when we ordered that Dinner for Two.

Hmmm.. such a strange ponderment.