Benefits based on service credited under this article and Article 4 (commencing with Section 20730), where the service credit is derived from a member’s redeposit of contributions, shall be paid from contributions of the employer or employers which is or are the source of the contributions redeposited by the member. The employer liability in this regard shall be limited only to its contributions and no employer shall be liable for any portion of the member’s own contributions. All employer contributions, for purposes of this article, shall be made by adjustment of the employer’s rate of contribution.