After performing ablution if someone asks me if I am in the state of ablution, can I say, “In shaa’ Allaah” (If Allaah wills)?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu 'alyhi wa sallam, is His slave and Messenger.

If you are certain that you are in the state of ablution, your reply should be in the affirmative, but saying, "If Allaah wills" in this case is inappropriate and does not make sense. Making things contingent on the will of Allaah applies to future events, not those in the past. Allaah The Almighty Says (what means): {And never say of anything, "Indeed, I will do that tomorrow," except [when adding], "If Allaah wills."} [Quran 18:23-24] As for things that have already taken place there is no need to attach them to the will of Allaah The Almighty. This would be nonsense because past events could not have taken place except by the will of Allaah The Almighty, and had He not willed them, they would not have happened.

Allaah Knows best.