Hi Terri!
The issue you raise is one of the reasons Bitcoin is failing at the mission to be a “store of value”. The price fluctuations make it difficult (some would say impossible) to fulfill this role.
In your example you would have to deposit more Bitcoin in the escrow account. If the price went up, the excess Bitcoin would be returned to you at closing.
If Bitcoin ever develops to the point where it is used in this way, the procedure will probably be to require a larger escrow deposit to accommodate market fluctuations. Again, any excess would be returned at closing.
I think this is pretty unlikely. The most likely positive development that Bitcoin can hope for these days is that people use it more like a bank account and cash out coins when they want to buy something. The actual use in day-to-day transactions requires a much more stable price than the market can realistically achieve.
Make sense?
Mark