I'm just a beginner so bear with me. Consider two frames at rest wrt to each other separated by distance enough for light to take a minute or so. At a given instant we create two large dipoles by some method. Electric field will start to travel at the time of creation at speed of light. But before electric field reaches frame 1, we reunite the dipole in frame 1. so when electric field reaches from frame 2 to frame 1, there'll be no effect on it, since net charge is zero. But electric field from frame 1 when reaches, creates force on dipole of frame 2. So in reality what happens? Is conservation of momentum violated? Or law of equal and opposite forces???
Action equals reaction isn't generally true in electrodynamics such as the simple case of magnetic forces acting upon moving charges, whereas the conservation of momentum holds in all areas of physics. There is momentum associated with the electromagnetic field which must be added to that of the momentum of the charges it acts upon to get the total momentum of the electrodynamic system.
Although the total momentum of any electromagnetic system is conserved, once you take into account the momentum stored in the field, as John McVirgo points out, the action-reaction law only really holds in electrostatics.
In fact, you don't even need the finite propagation speed of the field as in your example. Consider two equal positive charges moving towards the origin along the positive $x$ and $y$ axes. Then they experience magnetic fields in the positive and negative $z$ directions, respectively, and therefore magnetic forces in the positive $y$ and positive $x$ directions, resp., and these cannot cancel each other out. In a full description, this motion induces crossed electric and magnetic fields throughout space, which store the momentum not accounted for by the non-cancelling forces on the charges.