Tenant did repair, wants to deduct

[Bienes Raices]

The front doorknob broke yesterday and the tenant replaced it without telling me about it. He now wants to deduct the $20 from this month's rent for the cost of the doorknob.

This tenant is very handy, but the lease states that the tenant is supposed to call me and then either I do the repairs or get a contractor. It's only a doorknob, but I'm a bit uneasy about having one set of rules for one tenant and a different one for the others (the other tenants do not attempt repairs on their own, they always call). Am I being too uptight about this?