Hard wiring condensate pump with ground to furnace with only hot / neutral
Use a pigtail for the ground and a cord grip for the cord

With the power off:


Fit the cord grip (a Cooper/Crouse-Hinds NCG50-50 will do the trick) to a knockout in the box using a matching locknut and run the cut-off cord end through it, then tighten the cord grip down. This provides strain relief and sealing for the cord connection, which is permitted here by NEC 400.7(A) point 7 (preventing the transmission of vibration; in this case, from the pump to the rest of the furnace).
Wire the cord hot to the furnace hot, and the cord neutral to the furnace neutral
Remove the existing wire from the ground screw, and nut it together with a length of bare 12AWG as well as the ground from the cord
Land the length of bare 12AWG (this is a pigtail for the ground) on the existing ground screw in the box
Button it all up and turn the power back on