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Unfortunately, once you signed that lease, you obligated yourself to allow the tenant to stay for the whole term of the lease. An oral agreement is not legally allowed to change the terms of the written lease under the parol evidence rule. So, you are bound to honor the lease and you cannot remove through an unlawful detainer because they are legally in the premises under a lease. If you had included a written cancellation upon sale clause, then you could have terminated the lease upon your selling, but you did not do that in writing.
So, now you have two legal options, 1) pay them some number you can agree upon for them to leave or 2) sell the property subject to the lease and then the new owner has to deal with the lease and the tenants.