If the formatted copy of the field is a calculation field (or auto-entered calculation) referencing the unformatted field, You can give the formatted field an opaque fill color and place it on top of the unformatted field to hide it. You can set behavior options in the inspector to deny browse mode access to this formatted field. Then, when you click the formatted copy of the field, the unformatted copy hidden behind it will pop to the front and be accessible for editing. When you exit the field, it disappears back behind the formatted field which will update to show the formatted version of the data in the unformatted field.
The hidden unformatted copy can also be part of the layout's tab order so that you can also use the keyboard to exit the previous field and enter the unformatted field for editing and you'll see the same behavior on your layout as you do when you click the formatted field.
Thanks Phil. I will try get back to my desktop
This worked perfectly. It gives the illusion of masking.