What do you do when a tenant covers expenses?

[Lauren Daly]

We have a tenant who has a background in construction who moved in to our A class property about a month ago. He discovered a problem with the light fixture in the entry way and has offered to replace it with a similar light fixture that he found for $50 at Home Depot. We will go make sure it gets installed correctly, etc. but he asked us if he could reduce next months rent by the amount of the light fixture. 

Is reducing rent when the tenant covers expenses OK, or do I need to get him to pay the full amount and then write him a check for the light fixture?