Use a conditional value list. A simple form that relies on a calculation field (option 1 in the following link) can probably be set up to serve as the source of values for you value list. (Set up the calculation with an If or Case function to only return the value for a given property when it is vacant.) Or you may find that option 2, using a relationship, works better for you.
Thanks for all the information. However I would like to know about some easy to use and user freindly, beginner level software for rental managaemnt service. Please help me out. I want it as soon as possible.