Tenant problem with backyard and rent discount

[Chris Latham]

Help.  I did what I thought was a good thing that turned out to be bad.  We purchased a 2-story house in the Boise Idaho area that needed a lot work.  We put $20,000 into and made it really nice.  We got a great price on the purchase because of all the work needed.  Of course it took longer than we thought and our handyman / contractor ran way behind.  

We found a tenant quickly.  But we did not finish the back yard by the time the tenant moved in.  We cut it down and killed off all the wild grass and weeds.  It still looks terrible but much better than before.  We plan to completely redo it.  The yard is very large and it's going to take a lot of time and money.  We could have had in done in August but our handyman kept making promises that he could not keep.  Then a one week delay turned into two and on and on.  Now it's October and it's still not done.

Here's the punch line.  I promised the tenant a $100 discount on rent until the grass is in since we thought it was going to be done by the end of August.  She got her $100 rent reduction for August and September.  Now I have finally found someone to do the work.  But in this area "irrigation" water is free but it gets shut off for the winter in the middle of October.  (Less than two weeks from now).  If  we put in grass now, it won't be watered by the automatic sprinklers and will go dormant anyway.  I told her that we did not expect her to hand water it or move a sprinkler around.  

Now, she wants to continue the $100 discount since I sent her an email saying that the discount would continue until the grass is in.  I told that's unreasonable since we now have no way to grow the grass and she should start paying the $100 until spring when we can finally put in grass.  Now I regret having given her the $100 discount in the first place.   I should mention that the lease states that any changes to the lease must be in writing and signed by both her and me.  She says she talked to a lawyer and my email signature counts as a signature.  She plans also to do some sort of lawsuit because I did not live up to my promise.   What should I do?