Reference for Ketubot 215:2

בשלמא שוקל לו את שקלו מצוה קעביד דתנן תורמין על האבוד ועל הגבוי ועל העתיד לגבות

It makes sense that he may “pay for him his shekel” [because by this payment] he merely performs a mitzvah, for it was taught: One may withdraw [from the funds of the Temple treasury] on the account of that which was lost, collected or about to be collected;

Jastrow