Cal. CCP § 431.70

Where cross-demands for money have existed between persons at any point in time when neither demand was barred by the statute of limitations, and an action is thereafter commenced by one such person, the other person may assert in the answer the defense of payment in that the two demands are compensated so far as they equal each other, notwithstanding that an independent action asserting the person’s claim would at the time of filing the answer be barred by the statute of limitations. If the cross-demand would otherwise be barred by the statute of limitations, the relief accorded under this section shall not exceed the value of the relief granted to the other party. The defense provided by this section is not available if the cross-demand is barred for failure to assert it in a prior action under Section 426.30. Neither person can be deprived of the benefits of this section by the assignment or death of the other. For the purposes of this section, a money judgment is a “demand for money” and, as applied to a money judgment, the demand is barred by the statute of limitations when enforcement of the judgment is barred under Chapter 3 (commencing with Section 683.010) of Division 1 of Title 9.