It really is a sticky issue to some degree. On one hand if you return the money to the other famiy, they may feel insulted that you returned such a gesture they may feel obligate to give you. On the other hand, I understand the reasons why you did what you did, and it's obvious you did it with nothing expected in return.

I would though send a card expressing my sincerest thanks, stressing how much you appreciated their guesture.

A third option, assuming you aren't in need of the cash and far more drastic, would be to donate the $200 to a charity in the elderly couples name. It is afterall, money you didn't expect to receive. You could then send the note from the charity to the couple's home.