Allocating $ Paid by Tenant

[Matthew Ginsberg]

We have a tenant in Washington County, Maryland who pays his rent on time every month, but has refused to pay his water bill, even though in the Lease Contract it states he's responsible. Our Lease Contract doesn't currently say that all payments will be applied to non-rent charges first, oldest to newest, but it also doesn't say anything about how we may allocate the payments tenants make to us. Can we choose how we allocate the rent $ he pays us (meaning allocate the rent $ he pays us toward the water bill and the rest toward rent if we choose to)? Ideally, we'd like to do this so we can file for failure to pay rent against the tenant if it comes down to that, but we first want to make sure it's legal in our jurisdiction to do this. Any answers/ suggestions would be much appreciated!