Does touching one's wife with the tip of a fingernail before a kiss, to ensure no discharge has occurred during Tawaf al-Wada' after she felt some moisture, invalidate ablution?

If the private parts are touched only with the fingernail, this is not considered to invalidate ablution. The books of Hanbali, Maliki, and Shafi'i jurisprudence have stipulated that the touch that invalidates ablution is with the palm of the hand or the finger, not with the fingernail or the back of the hand or anything else.