Tenant paints house -now wants rent reduction

[Marci Stein]

tenant informed he painted the house and wants to pay half the rent  (400 not 800). 

He reminded me that when he moved in last year I said if he paints i will reduce rent, 

but never said how much. This was to be a discussion before-- not after- the paint.

If he had asked me before I would have said i will paint house (I really need the rent money.)

 BUt he took it upon himself. Do I now have to take money from the rent because he did the 

work, even though I never asked him to?