Best way to permit a tenant to break the lease if he wants to?

[Kay March]

I have two very needy, nitpicking tenants who have recently informed me that they would like to have the ducts cleaned and they are willing to pay for it. I have refused their request for reasons that I need not go into here. I believe they will continue to make an issue of the duct cleaning, claiming that it is necessary to cure their "allergies" (I don't believe they have allergies). At this point it is just fine with me if they wish to move out, and I plan to tell them in writing that they may break the lease and move out whenever they want to. Is there some reason why I should do more than send an email to that effect; for example, is there some reason why I should present this offer as an amendment to the lease?