If a newborn’s parents are indigent, the government of the District of Columbia shall pay all costs related to screening under this subchapter. If a newborn’s parents are indigent and the child’s residence is in the District of Columbia, the government of the District of Columbia shall pay any subsequent costs for follow-up testing and treatment. The Mayor shall define “indigency” under this section and may establish a sliding scale of partial payment by the District of Columbia government based on the parents’ reasonable ability to pay some of the costs.