Can I demand the return of the deferred dower from my ex-wife who absolved me of it at the time of divorce, especially if she only waived the "mut'ah" (consolatory gift)? And am I considered to have been deceived in this procedure? What steps can I take to prove her receipt of the amount? And is what the ma'dhoun (marriage officiant) said true, that she cannot be taken back except with a new contract and dower?

If the divorce is based on mutual release (Ibra'), then the wife has no right to take anything from which she released the husband, and she must return what she took. The husband has the right to demand it, but if he pardons her, it is closer to piety. Divorce by mutual release (Ibra') is a form of Khul', and Khul' results in an irreversible divorce (Talāq Bā'in), after which the husband is not permitted to take the wife back except with a new contract and a new dowry.