First, I need to add the typical disclaimers: that I have not reviewed the written lease agreement and cannot make specific comment to it, but only within the parameters of your question, that we have not discussed this issue and that a full discussion of the facts may well change my comments.
Ordinarily, the terms called for in the written lease agreement will be the ones enforceable. If there is any other documentation calling for the additional charge as part of the renewal agreement, that might well be considered a part of the lease "contract documents", particularly if it dates at the approximate same time as the lease renewal date.
A second factor is how long you continued to pay the additional $40.00 before you discovered the mistake. The landlord may take the position that your payment of the amount evidences your tacit understanding that the extra fee was acceptable. In other words, why didn't you check the lease before continuing to pay the additional amount? Ultimately, that is a question you will have to answer.
Finally, consider the potential gain you MIGHT realize against your detriment if you lose. At $40.00 a month, you may reach the point of diminishing returns quickly.