The tenant may be in violation of the lease; if the premises were clearly rented as a residence, not as a commercial or office setting, then it may be that an implied term of the lease was that it would be used for residential purposes only, and that therefore running a business from it may be a breach of the contract. Alternately, it may be considered that there was no meeting of the minds in forming the contract, since you were intending to rent out a residence and this person wanted to rent office space or run a home-based business from it; if there was no agreeement or meeting of the minds, there would be no valid contract formed. In either event, you may have grounds to terminate or void the lease, and if you can, then you could renegotiate a new one. You should consult with a landlord-tenant attorney about your options.