There are two phrases that should warn you when someone wants you to borrow something. The first is: "it's hard to find, but it's the best." The second is: "it's my favorite."
I had decided to fly from my home near Frankfurt to London with an empty suitcase and fill it with tomes from London's great used book stores. This friend, an oil company executive, said I had to take his hard to find guidebook. I said thank you, but no, in the nicest way possible, but he insisted, acting as if I was turning down his loan of friendship.
Well you know what happened. Somewhere in Heathrow or on the tube it disappeared. (It's really surprising that individuals don't disappear forever, given how poorly Heathrow is run.)
When I returned, he said" "you knew that was my favorite. It's very hard to find." After that our relationship ceased and we avoided each other at gatherings. There was no way I ever could have been contrite enough.
For women, I would advise never borrowing someone's favorite dress. For guys, don't ever borrow a friend's chainsaw. Count on your relationship suffering when the dress is forever stained, or the chainsaw bites into metal.