According to Agha Sistani, can the divorce of Khula be reversed with mutual consent of estranged husband and wife through some means – let's say the husband returns the money to the divorced wife and convert the divorce into a simple divorce where the two can marry immediately? 
And does this have any time limit to it, Agha, meaning the divorce cannot be reversed after passage of certain time period?

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If she accepted returning the money within the Edda period, then it automatically becomes simple divorce, and he can return her to the marriage without a need for another contract.
However, after the Edda period, they can start new contract, with no need to return the money, but he to pay new dowry.