Display Screen or Portion Thereof with a Transitional Graphical User Interface

FIG. 1 is a front view of a display screen or portion thereof with a transitional graphical user interface;

FIG. 2 is a front view of a second image thereof;

FIG. 3 is a front view of a third image thereof; and,

FIG. 4 is a front view of a fourth image thereof.

The outer broken lines in the figures show a display screen or portion thereof, or an electronic device having a display screen, and form no part of the claimed design. The other broken lines in the figures show portions of the graphical user interface that form no part of the claimed design.