I would totally return the item !
I have lost my purse and wallet,debit or Credit Cards each at least a dozen times and each time someone was kind enough to return it to me or to the store manager where I claimed it.

You never know what that $ was for like others said, Maybe the couple you returned did look well off but maybe one of them is going to lose their job soon, etc.. you just never know.

When I was 13 I got paid for my summer job all in cash and in a lump sum of $2000.
Up until that day , virtually all the clothes I had ever worn were hand me downs.
I decided to go buy myself some clothes so I shoved a huge wad of cash in my back pocket and stuffed the rest in my purse.

I had been shopping for a while when an elderly woman ran up to me looking frantic !
She had a handful of $20 's and said I was dropping them all over the mall !!

hahaha, I felt like an idiot but was soooooo thankful that she was honest.