Relying on Lease Terms to Invalidate Oral Consent to Sublet

Q A medical clinic tenant in my strip mall renewed its lease only because I orally agreed that it could continue to sublet a portion of its space to a blood lab that serves the clinic's patients. Now, I have a lucrative offer from a franchised blood lab that's a competitor of the clinic's lab. But the franchised lab won't rent the large space it's interested in from me as long as the clinic's lab is operating.