How does a wife return her dowry to her husband if he requests khulʿ, when her dowry was his teaching her surahs from the Noble Qur’an?

If the wife's mahr (dowry) was memorizing the Qur'an, and she completed its memorization, then they agreed to khul' (divorce at the wife's instance) in exchange for her returning the mahr, she must return the cost of the memorization. Experts in the field estimate a suitable wage for what the wife memorized. The evidence for this is that jurists mentioned that if he divorces her before consummation after teaching her the Qur'an, he reclaims half the cost of the teaching.