Reducing rent for managing the yard?

[Marty Bauer]

Recently purchased an out of state duplex and one tenant pays on time but is paying under market by $100/mo. 

We are coming up on his lease renewal and he offered to maintain the yard (cut grass, clear snow, trim bushes, etc) in exchange for keeping rent at the same rate he has currently. 

The yard maintenance would otherwise be $80/mo to hire it out. 
I'd be down a net $20/mo but keeping a tenant that pays on time and otherwise keeps the unit clean and treats the property as their own.

Have you swapped rent for yard maintenance before? If so - have you had good/bad experience with it? What holes should I be looking out for? 

Thanks!