If they have shown you a written ijazah of them being able to receive khums from the Marja', or you are of the level of trusting the individual in what they are saying about their receipt and its authority, then that would suffice.
If not, request a receipt from the office.
The requirement of A.U. Seyid Sistani's office is that whenever you give a khums payment, a receipt must be issued from the office.
With prayers for your success.