Anyway if we consider the collision as inelastic (at least partially inelastic as I don't think that the bat could catch the ball and fly away with it at a slower speed...
) part of the original kinetic energy is lost and thus the internal energy of the system should increase (I mean: in the collision, if we don't consider it as a perfect event, as that of two solid iron balls thrown one against the other at fixed speed and with a contact time →0, atomic interactions cause a small part of the total energy to be lost and this energy increases of a really tiny amount the temperature of the bodies involved and of the environment too, even if usually we don't consider the last one to simplify the situation).
I remember a few problems I did some time ago in which you were also supposed to evaluate the energy lost, given the masses and the speeds before and after the collision, and thus the final temperature of the system (maybe taken from the Halliday-Resnick-Krane, try to check the thermodynamics section on it if you can).
Obviously I could be wrong, but if you consider that the energy has to remain constant you can ask yourself where the part lost will go...