Yes, there is. Press and hold SHIFT.

To erase - select object, click Erase Tool, press and hold left mouse button, 'paint' over the part you want to erase.
To unerase - select object, click Erase Tool, press and hold left mouse button, PRESS AND HOLD SHIFT KEY, 'paint' over the part you want to unerase.

UN-erase, De-erase, Anti-erase, Reverse-erase? Non of them seem like real words