פירוש על עבודה זרה 125:8

According to the first baraita, if he has sex with her and then gives her the animal, the animal is permitted, whereas the second baraita says that it is prohibited. R. Nahman b. Yitzchak resolves them by saying it depends on how he makes his statement to her. If he specifies beforehand that this lamb will be her payment, then it is prohibited. But if he does not specify which lamb then when he had sex with her, that lamb was not designated as payment. The lamb can therefore be used as a sacrifice.