Hajjaj bin Hajjaj Aslami narrated from his father (Sayyiduna Hajjaj, may Allah be pleased with him) that he asked: O Messenger of Allah! What thing will remove the right of milk from me? The Prophet (peace and blessings be upon him) said: "A slave or a slave-girl."
(Commentary on Hadith 2290) That is, purchase a male or female slave for the foster parents so that they may serve them; in this way, the right of fosterage (haqq al-rida‘ah) will be fulfilled.