Do you already have a VFD on it? If not, you can't just hook up resistors and expect it to stop very much faster. Dynamic Braking involves keeping the motor energized to make it into a generator, then pulling the kinetic energy of the spinning mass out of that generator and dumping it into resistors as heat. a VFD is capable of doing that. If you just open the contactor and then close another contactor afterward that connects the resistors, the motor magnetic fields have collapsed by then and all you are left with is a little residual magnetism. That's where you are getting the 20V you see, but the energy that this represents that would be useful toward slowing the motor is minuscule, it would likely not even be noticeable.
If you don't already have a VFD, you can't do dynamic braking. If you have one, it might already be capable of it, but you MUST get the resistor information from THAT drive mfr, otherwise you can burn up the braking transistor. If you have a VFD that is not capable of DB, then there are retrofit units from Bonitron that can be added, and they will give you the resistors or the values.
