Is a secret agreement between spouses to drop the deferred dower (mu'akhar al-sadaq) documented in the marriage contract considered a deception (tahayul)? Is the deferred dower a debt that is religiously obligatory to be paid? Is it permissible to renew the contract to conform with this agreement?

The deferred dowry that is due is what the spouses secretly agreed upon, not what was written. The dowry is a right of the woman, and she has the right to relinquish all or part of it as long as she is an adult of sound mind, based on the Almighty’s saying: "Unless they [the women] give it up" and "But if they, of their own good pleasure, remit any part of it to you, take it, and enjoy it with good cheer, agreeable." What you did is not an unlawful deception. If the return to the husband is during the 'iddah (definition) (waiting period), her family does not have the right to refuse, and it takes place without a new contract or dowry.