A differential transmitter (usually) has the two signal lines with voltage v/2 and –v/2 with respect to the ground.
The question is do you connect the ground of the differential part to the ground of the single-ended part? (or are they connected somehow?)
If not, and one of the two grounds it’s a floating ground, then you can connect v/2 at the single ended signal line and –v/2 at the single ended ground and it should work well except some loss at high frequencies due to the mismatch between the field configurations in the balanced and unbalanced parts.
However, this is not the usual case, the grounds are connected somehow, even if you don’t know.
Then you can connect one of the differential lines to the single ended and you have only half the voltage at the receiver (careful with the impedance matching here). Also to avoid strong reflections coming back to the transmitter you should terminate the unused differential line.
If you connect one of the differential lines to the single ended line and the other to it’s ground then in this case you are shorting one voltage and have strong reflections back to the transmitter (and still getting only half voltage at the receiver).