The two year lease is actually a one year lease unless the tenant's signatures were notarized. The Court will reform the document because it is not possible to convey an interest in real property lasting longer than one year without acknowledgement. Alternatively, the Court could determine that all you conveyed was a month to month agreement and the lease is invalid all together.
Thus, they are asking you to change a one-year lease (through Feb 2010) to a six month lease. If you agree, it can be done. If you do not agree, then their offer to pay you an additional month's rent is substantially similar to the relief you would be given by a court, assuming arguendo that the Court reformed this to a one year lease, not a month to month rental agreement, with the advantage of not actually having to go to court and litigate the matter.
Yes, you do have to mitigate your damages, i.e. make an effort to re-rent. In today's economic climate, a $100 reduction in rent seems like a cost of doing business, and isn't strictly speaking your tenant's fault. Thus a court is not likely to award you the additional $100 per month that apparently you will not recieve.
You do not have to accept any prospective tenant, no matter what they offer to pay you, so long as you reject a tenant for valid reasons and do not engage in impermissible discrimination. Presuming this is a one year lease, you have the "right" to sue your current tenants for every dime of rent they owe you between now and February 2010. The real question you might want to consider is: How likely are you to prevail, and if you do prevail, do the tenants have sufficient resources to pay off a judgment or are you simply going to be forcing them into bankruptcy?
If they elect to bankrupt out of this debt, it will be completely, absolutely uncollectable.
That is the real question. Hope this helps you consider your course of action. Elizabeth Powell