Even IF you struck the warranty provision in the lease, you are still bound to return the money. There is an unpublished case just like that and I will look it up and post it here.
The covenant of seizin, when there are words of grant (such as the grants, lease, sets over, etc in the lease) promises that you have the authority to lease.
Striking the warranty does no good to allow you to keep the bonus money. You would need a broad disclaimer to do that, which states that any monies paid will not be returned in the event of title failure.