Is the Dawn Prayer to be repeated if a drop of urine is found on the barrier after its time has expired, and should it be repeated immediately or with the Dawn Prayer?

What is found on the barrier after the prayer does not affect its validity, as it might have descended after the prayer. Your prayer is valid and you are not required to repeat it. And whoever is required to repeat a prayer must do so immediately, even if it is during a time when prayer is prohibited, because the Prophet, peace and blessings be upon him, said: "Whoever forgets a prayer, let him pray it when he remembers it; there is no expiation for it except that."