If your Landlord did not make up with a written contract a late payment of 100 bucks then he cannot charge you that. Also if you paid 875 instead of 850 then you included the returned check fee so he should not be charging you an additional 25.00
There is no sure way of telling whether the landlord is right or wrong. The amount charged for late payment depends on the contract you had with the landlord. It would be wise to seek legal advice on the issue from a lawyer.