Cal. Health & Safety Code § 127440

(a) The hospital shall reimburse the patient or patients any amount actually paid in excess of the amount due under this article, including interest. Interest owed by the hospital to the patient shall accrue at the rate set forth in Section 685.010 of the Code of Civil Procedure, beginning on the date payment by the patient is received by the hospital. However, a hospital is not required to reimburse the patient or pay interest if the amount due is less than five dollars ($5.00). The hospital shall refund the patient within 30 days.
(b) The hospital may reimburse the patient, but is not required to do so, if the hospital or the department determines that a patient qualified for financial assistance at the time the patient was first billed and either of the following has occurred:
1. (1) It has been five years or more since the last payment to the hospital, hospital assignee, or debt buyer.
2. (2) The patient debt was sold to a debt buyer in accordance with state law in effect at the time the debt was sold, if sold before January 1, 2022.
(c) This section does not diminish or eliminate any rights or responsibilities a hospital may have, nor any rights that a patient may have under existing federal and state laws, including, but not limited to, 26 CFR Sec. 1.501(r)-6.