The source major power can reverse this process [lend aircraft] during this step if the striped aircraft is in the force pools or reserve pool. Move it back to the lend lease pool and move the matching aircraft to the source major power’s force pool.
Some lend lease aircraft are good. Others, on the other hand, aren't. Thus, if one power is building one type of unit, and the other isn't, one can try to arrange it that the bad units are in the force pool of the power that is not building that sort of unit this turn.
This leads to the question of exactly when one can lend units. MWiF allows you to do so at any time, but this is not quite right by the rules; technically it should only happen immediately before the build units step. The important point is that bad planes must be in some power's force pool during the build units step; one can not juggle things so that no the units are not in anybody's force pool if everyone is building that type of unit.
