Changes to a Commercial lease

In most cases, general partners (as opposed to limited partners) have authority to bind the partnership.  Even if, in this particular case, your partnership agreement provided that your partner did not have authority to agree to the modifications, your remedy will probably lie against your partner, not the landlord, as the landlord will almost certainly be able to rely on what is known as the "apparent authority" of your partner to bind the partnership.  Apparent authority means that when you have given someone the indicia of authority other people are entitled to rely on it.  Since your partner is a partner, he or she would generally have authority to make decisions for the partnership, so unless you specifically told the landlord that your partner did not have that authority, the landlord is entitled to treat your partner as if he or she was authorized to modify the lease.