Property should not be change by the component, as they are own by the parent/exterior of the component.
To implement your pattern (two ways binding) you should emit a trigger (you can call it onChange for exemple) and update your variable using a workflow on your instance.
You can also expose a workflow to modify an internal variable from outside.
Other option
If you wish each instance to always have this value, you can define an internal variable and expose it, and have only an initial value property.