Is it permissible for a charitable organization to stipulate that a needy person waive any amount exceeding their needs in favor of the organization's fund, and what is the legitimate solution for accumulated surplus funds?

There is no religious impediment for the association to stipulate that patients relinquish any remaining donations from their treatment for the benefit of other similar cases. They are bound to fulfill this condition based on the Prophet's (peace and blessings be upon him) saying: "Muslims are bound by their conditions, except for a condition that forbids what is lawful or permits what is unlawful." As for the remaining donations for the patient's treatment, it refers back to the intention of the donors. If they intended to transfer ownership of the amounts to the patient, then the remainder belongs to him. If they intended for it to revert to them or to the association, then it reverts to what they intended. If they did not specify anything, then it refers back to custom.