מדרש על ויקרא 14:28

10) (Vayikra 14:17) "And from the rest of the oil which is on his palm, the Cohein shall place … upon the blood of the guilt-offering": (Is the meaning that the blood precede the oil, or that if blood is there, he shall place (the oil), and if blood is not there, he shall not place (the oil)? It is, therefore, written (Vayikra 14:28) "upon the place of the blood of the guilt-offering." It is not the blood which is the decisive factor, but the place.