How to unreserve the purchasing document in the same period in which the document originally got reserved?
As per the functionality, when an unreserve action is taken in a purchasing document, encumbrance reversal will happen in current GL period considering the system date. If you want the reversal to happen in the same period in which the document originally got reserved, you can use the check box "Use Document GL Date to Unreserve" so that the reversal will happen based on GL date entered in PR/PO distributions.
This check box would be available only when doing Unreserve/Cancel/Final close where you can manually enable it and get the encumbrance reversal in the same period in which it got created.
But if you perform a Reject action on the PO or Return the PR or delete the lines which are in Incomplete,Reserved status, this check box will not be available. In such cases, you may need to set the profile option “PO:Use Document GL to Unreserve” to Yes so that the encumbrance reversal will happen automatically in the same period in which it got created.
This unreserve action or the encumbrance reversal will happen based on the GL date of purchasing documents only when that past PO period and past GL period is in Open status. If that PO/GL period is already closed, then the reversal would happen only in the current period based on system date in-spite of this profile option being set to Yes.