But I need something you'll come back for

I accompanied my wife when she went to get her hair cut. 

Reading a magazine, I found a hairstyle I liked for myself. 

I asked the receptionist if I could take the magazine next door to make a copy of the photo.

"Leave some ID, a driver's license or a credit card," she said.

"But my wife is here getting a haircut," I explained.

"Yes," she replied. "But I need something you'll come back for !"