The prescribed ablution removes the state of ritual impurity, even if the intention contradicts it

If a person performs ablution for the purpose of sleeping and intends to sleep, but when he is about to sleep he remembers that he must perform the witr prayer before sleeping, may he perform the witr prayer with that ablution?
Yes, if a person performs a prescribed ablution, whether for sleeping, for reciting the Qur'an, or for something similar, and then he intends to pray with that ablution, there is no harm in it. This is because when he intended the prescribed ablution, his state of ritual impurity was removed, so he may pray as much as he wishes.