From the Navigation Pane
Expand                   Self Service
Expand                   Setup
Double click on       Process Administration
Search                    for the Leave transaction in question
Click on                  the Leave Transaction in question

You will notice that once the Approver approves the leave it remains Partially Approved as below:

From the Navigation Pane
Expand                   Self Service
Expand                   Setup
Double click on       Process Definitions
Click on                  the Definition in question

If this is the case where you have a Process Definition setup as below with one Reports to Approver:

From the Navigation Pane
Expand                   Parameters
Expand                   Workflow
Double click on       Approver Management
Click on                  the Approver Group in question

You will notice that there will be a Min Approver Account of 1 which you need to remove that will correct the leave approval going forward.

Any backdated Partial Leave approvals can be done in the Process Administration.