California

I had a tenant rent my condo and sign a month to month lease after viewing it. She gave notice and moved out a few days later. She return the keys and agreed verbally that our transaction was complete. Rent had been paid for the full month on the 1st. If I am able to re-rent the condo a few days before the end of the month, am I legally obligated to refund a pro-rated amount if it is less than the expense I incur in re-renting the condo?

Her representation of herself (which caused me to pass up the other two people who wanted it) proved inaccurate.

A lot of that would depend on the wording of your lease, I believe. It doesn't sound like she gave proper notice (less than 30 days).

The best policy (and law in many, if not all places) is not to "double dip". I don't believe you should keep her "rent", if someone else is paying rent for those same days.

You may or may not be able to apply it to damages you suffered as a result of he breaking her lease. That would depend on your lease and your state's laws.