Is what the husband did—using a sum of money sent to him to support Quran memorization circles, then investing it in a personal project after the circles stopped, and buying Qurans and Islamic booklets to repay the remaining amount—correct, or is he indebted to the Sheikh?

The husband was obligated to spend the donated amount on the charitable causes specified by the donor. It is not permissible for him to invest it in his private project without the donor's permission, as that constitutes a betrayal of trust. Since he has fully repaid the amount, he owes nothing further except to repent to Allah Almighty for this action.