On the off chance
This is a very common expression in everyday conversation and is also suitable for writing, except in very formal contexts. It is used worldwide in the same way. See http://fraze.it/n_search.jsp?hardm=1&t=0&l=0&p=2&q=on+the+off+chance
Here is an example sentence:
"I will take an umbrella to work with me on the off chance that it rains."
The meaning is the same as:
"I will take an umbrella to work with me in case it rains."
Perhaps you have seen in the weather forecast that there is a small chance of rain during the day. So you are taking the precaution of bringing an umbrella with you. It is not a conditional situation (e.g. If it rains, I will bring an umbrella). It is a definite action (I will take an umbrella) which has a reason (on the off chance/in case it rains).
Can you make up your own examples with the following structure:
a) Decision/plan (clause with will or going to future) +
b) on the off chance that +
c) Reason (present simple or present perfect clause)
Good luck!
Good efforts, Mumtaz and Suade.
Mumtaz, your example works if we have other context which tells us that you were going to be in an area where you could possibly buy nice things for your house.
Suade: 1. I presume Emily left her wallet with you some time before!
2. This doesn't really work. Aerosoles is a commercial operation. It sells boots to people who want them. Here's a related example that might help you understand the meaning:
"I know you don't normally go shopping on Wednesdays, but on the off-chance that you go shopping today, can you buy me a particular model of shoes from Aerosoles?"
3. This works. The police didn't expect to find her but they did everything possible.
I cashed my paycheck today on the off chance that I would find a nice thing to buy for my house.
I hope it sounds right :')
