Lease issues: We brought in a pet and signed a pet addendum

We signed a lease in April of 2009, with a one year term. In May of 2009, we brought in a pet and signed a pet addendum. The pet addendum states that there is a $150 pet deposit and a $20/month pet rent. The manager at the time waived our pet rent and initialed it on our pet addendum. Now, we have a new manager and she is telling us that we have to pay the pet rent because the manager before her had no right to waive it. In my opinion, the pet addendum was signed by the manager at the time and is legally binding. Can they force us to pay the additional $20/month even with the documentation that we have?

I would say no. The addendum is part of the lease. You have a lease in force which cannot be changed. However, your lease ends in only a few short months. Expect to be served notice that your rent will increase or the pet fee will increase at the end of your lease.