Reacting...fast or slow

My son came home from school without his water thermos, but with two frogs!  I was more concerned about the frogs rather than where his thermos was. Suddenly, I got a phone call.  I wasn't expecting it, so it took me a minute to tune in to what the person on the other end of the line was saying.  It was son's classmates mother. I caught that she was talking about the thermos and misunderstood the next thing she said.  I thought (I guessed) she said my son left it by the fields and so her son brought it home.  I missed some of what she said until she started to say that she could find the broken part online perhaps rather than replacing the entire thermos, since it's quite expensive.  This is when I clued in. 

I had to ask her to hold while I asked my son what happened.  I clarified which thermos it was and what the real story was (the quick version anyway).  According to him, the classmate was passing the thermos to him, but due to the weight of the water inside, it slipped from his hands and then the cap broke.  

The cap was already partially broken.  I knocked the thermos over myself a couple of weeks ago and a piece of the cap broke off, but it could still be used. I couldn't let this boy be the scapegoat for the broken thermos nor ask for any money for it.  It was an accident. I thought sending it home with the other child was a bit severe.  I would have accepted my sons explanation and carried on.  On the other hand, it teaches children to respect other people's belongings.  I was worried saying not to worry about it would cause more problems, but the thermos was truly on its way out.

I told the other mom not to worry about it. When she asked what to do with the thermos I said we would take it back as is (worried it would be a burden to recycle).  I thought she would send it back to school with her son.  The next thing I know the mom is at my door with the thermos and a bag of bread from her husbands bread shop, apologizing for the trouble.

After all was said and done, I asked my son to be very careful touching his classmates belongings.  If I have to react this swiftly with an apology and a replacement, I'm in trouble!  It takes me a little longer to sort such things out and to decide what the appropriate action/reaction should be.

What would you have done?