Here is a case from the United States Supreme Court that might be of some help:
In Bearden v. Georgia, 461 U.S. 660 (1983), the Court addressed the due process and equal protection concerns of imprisoning an individual for nonpayment of a fine or restitution. The Court recognized that differential treatment of indigent defendants in revoking probation may violate the equal protection clause and that the fundamental unfairness of revoking probation based on an indigent's failure to pay a fine raises due process concerns. Thus, “if the State determines a fine or restitution to be the appropriate and adequate penalty for the crime, it may not thereafter imprison a person solely because he lacked the resources to pay it.” Conversely, however, nothing precludes the state from imprisoning a defendant who willfully refuses to pay a fine or restitution. The critical factor under Bearden is the reason for nonpayment. If the probationer is without fault, revocation is inappropriate.