There are a few stages to resolving this issue.
To get to the first session:
Open Financials to Toolbar.
Click Sales/Purchase Ledger.
Click Customers/Suppliers.
Click the Transactions tab.
Find and select the transaction to be de-allocated.
Right-click and select Matching History.
Right-click in the window and select Next Session.
Repeat Step 6 until the option is greyed (this will be the 1st allocation session)
To de-allocate the transaction:
Follow the above steps to locate the 1st allocation session.
Right-click in the window and click De-allocate.
Click Previous Session.
Repeat step 3 until you find the next allocation session (de-allocation sessions are indicated by red writing at the bottom of the window).
Right-click and click De-allocate again.
Repeat steps 2 to 4 until all of the sessions that need to be de-allocated have been.
To re-allocate a session that was de-allocated by mistake:
To locate the session in matching history, click Next or Previous.
At the bottom of the window, check it says De-allocation session in red.
Right-click and select De-allocate. This will effectively reverse the de-allocation.
Note: If a de-allocation session is de-allocated as this actually re-allocates the value and then ends in a loop of de-allocation sessions.