The issue of an existing lease(s) is one that should be addressed in the purchase and sale agreement. Along with issues like allocation of rents (depending on when closing occurs), security deposits (will you get them? will you get a credit for the deposit(s)?), past due rents (who has to collect them? who gets them?) and proper notification of the tenant that you/your company is the new "landlord", modifying the leases while your sale is pending or not (e.g., can the seller decide to renew the existing lease? can he/she forgive past due rent? reduce rent? can he/she sign a new lease with a "friend" for much less than market rent?) should all be addressed up front. Buying real property with tenants/leases requires substantial due diligence--you are buying not only "property", but a "business" with cash flows and assets (the leases). Your lender, if any, will also be interested in knowing what the leases say.
You don't mention if the existing tenant has a renewal option (with set renewal rates) in his/her lease, which would affect the answer--you may have no choice in the matter. It is disturbing that the issue of the existing tenant seems to be a surprise to you and that the seller "just informed" you about the tenant.
With respect to the existing lease, if there are no renewal options, you should be able to offer a new lease term to the tenant at the rates you wish for--and he/she is free to refuse and move out at the end of the term. You may want to require that the current owners obtain a signed renewal of the lease at rates you find acceptable--built in cashflow. You may also want to make sure the seller sends notice of termination of the lease. There may be holdover rental rates in the lease that apply. You may also have to deal with an eviction. In my experience, tenants will exploit the "changeover" period if they can, so it is important to know what your--and the tenants--rights are AHEAD of time.
Counsel from an experience real estate attorney is likely essential here as the documentation you receive and what you sign at closing will be controlling. Conducting proper due diligence before you are stuck with a property and tenant is critical.
John M. Erdek, Esq.
Peak Legal Group, Ltd.
**DISCLAIMER: The response above does not form an attorney-client relationship nor is it intended to be anything other than the educated opinion of the author. It cannot be relied upon as legal advice. The response given is not intended to create, nor does it create an ongoing duty to respond to questions. The response given is based upon the limited facts provided by the person asking the question; to the extent additional or different facts exist, the response might change. Responses are based solely on Pennsylvania law unless stated otherwise.**