EL expression to make DFF Read-only in payment screen

We have created a Descriptive Flexfield (DFF) named "Entered By" on the Payables Payment page in Oracle Fusion.

Requirement:
The DFF should be editable when creating a new payment.
Once the payment is saved, the DFF should become read-only (not editable).

what binding or EL expression should be used to make a Payment DFF read-only once the payment is created and saved.

Joseph