Negative mass exerts negative gravity, which exerts a negative pull on a positive mass (which sums to a push). Positive mass exerts positive gravity on a negative mass, which, once you work through some math, sums to a pull.
The end result, if you position a positive mass near a negative mass, is for the negative mass to push the positive mass, and the positive mass to pull the negative mass, which produces indefinite, "0 energy transfer" acceleration without a change in the momentum.in the system. The end result is that, after an arbitrary time, the system has achieved an arbitrary velocity, with zero change in momentum.
Also, if they ever touched each other, the positive and negative mass in contact would annihilate each other without a release of energy.
The math is left as an exercise for the reader; I've had my fun here.
Or you could Google it.
However, negative mass is "exotic matter" and there is no known means to achieve exotic matter without exotic matter. However, negative energy *may* be possible, and matter is just energy in a stable form...