Unlike regular purchase invoices, you cannot unallocate quick entries from the quick entries area. Instead, you must unallocate from the supplier payment. 

Once you unallocate your transactions, the purchases quick entry invoice becomes outstanding and your supplier payment shows as a payment on account.

Unallocate from the supplier payment

1. From Banking, open the bank account with the allocated payment.
2. Find the supplier payment in the bank Activity.
3. Select the Supplier Payment to open its details.
4. Untick the checkbox on the invoice you want to unallocate. The transaction type on the quick entry invoice shows QE-Inv.
5. Select Save, then from the Confirm window select Yes to save the unallocated amount as a payment on account.