Legal Question in Real Estate Law in California

First, you didn't say whether this is residential or commercial property, or maybe something else (agricultural). The rules are mostly the same, but there are some differences, including the applicability of local rent-control and tenant rights ordinances. I'm going to assume its is a residential lease and that there are no contrary rent-control provisions.

Next, as I suppose you are aware, selling a property does not disturb the rights of a tenant, unless the lease contains a termination-on-sale clause (which is rare). All that happens is that the tenant gets a new landlord.

What you are proposing to do is, in effect, to modify an existing lease to add a termination-on-sale clause with a 30-day notice provision. I suggest giving the tenant a small consideration ($20 ?) in exchange for his promise to allow termination and surrender possession upon 30 days' notice of a pending sale.

The modification agreement should be in writing and signed by both parties. It should make sufficient reference to the lease and the premises (address) so a court could tell what it was in reference to. Also, the modification should make it clear that the tenant is also relieved of obligation to pay rent once he vacates following notice to do so.