When we rent, we do some things to try and retain good tenants. When they renew their annual lease, we time an upgrade or tenant request with the renewal. One year it might be as new refrigerator, another time new carpet or Re-paint deck, etc. I usually keep it around $500 or so, but have gone higher for good tenants. The repair or item is always tax deductible so it helps me out, but also keeps the tenant happy.
In this case, I would buy the washer and dryer, that way I would own it and can write it off. If you let them buy it and lower the rent to accommodate it, you can't write that off.