In all it's glory then.

There is a tension force on the block due to the motor. (\(\displaystyle T_{bm}\)) Newton's 3rd says there must be a force acting in the opposite direction acting on the motor. (\(\displaystyle T_{mb}\))

For the motor not to move it must be connected to the ground (table, whatever). Because the motor is not accelerating we can say, from Newton's 2nd that there exists a force equal and opposite to \(\displaystyle T_{mb}\) acting on the motor due to the Earth: \(\displaystyle F_{mE}\). (This is NOT a third law pair to \(\displaystyle T_{mb}\) even though it is equal and opposite to it.) Since there is a force on the motor by the Earth, the Earth must have a force on it by the motor \(\displaystyle F_{Em}\) (not shown in diagram.) Since there is a force on the Earth the Earth's momentum is changing, etc.

-Dan