option for tenants who purchase a home?

As a landlord, we would like to put a clause in for tenants who find a home to purchase - that they may break the lease provided we are provided with notice. We had that issue when we bought our first home, we found a great one, and had 6 months left on our lease (the last 6 months of a 3 year lease, too). The landlord would not budge on letting us out of the lease, or even reduce the number of months left.
While we do understand to a degree, it would have been nice if the landlord had worked with us on the issue.
We would like to offer our tenants an option to get out of a lease only if they have closed on a house, and can provide paperwork of such. Do any of you do this, and how do you work it into the lease?

We do this. We have a paragraph in the lease that basically says that the owners encourage the tenants to aspire to home ownership and will release the tenants from the lease with proof of a contract to purchase a property and at least 30 days noticeā€¦