The school's payment settings can offer an offline method alongside card payment. The buyer sees the chosen instructions in the offer presentation. When money arrives outside Cohorte, authorized school staff record settlement against the appropriate instalment in Sales → Payments. Verify the external payment first; an instruction shown to a buyer is not proof that funds arrived.
The public API's Offer payments endpoints control automatic charging and manual card charging. Offline settlement is a school screen workflow, not a public API endpoint.