Immediately after a TLI burn, the craft is not moving much radially; an ideal impulse would be tangential to the circular parking orbit. 400 m/sec in the opposite direction to the TLI burn would not return the craft fast, but it would surely cut the apogee of the orbit to a lower altitude than the nominal transfer orbit and return it to low Earth orbit altitudes sooner than that orbit would have.
There is a catch though. Apollo missions did not use Hohmann orbits, which have an apogee just tangential to Luna's orbit, for two reasons. They used more energetic orbits with apogees far beyond the Moon's orbit, partially because this would speed up the transfer to Lunar distance--the bigger orbit would have a much longer period than the Hohmann orbit would but the Apollo would travel only a fraction of the half-orbit before encountering the Moon. The other and even more important reason was to allow a "free return orbit," or after Apollo 12 anyway, one near one. For complicated reasons boiling down to the fact that the Moon's gravity, close up to it, changes the orbits, a Hohmann orbit is not free-return. The more energetic Apollo orbits were chosen because on an unpowered close encounter, the effect of the Moon would be to throw it back toward Earth on a similar energetic orbit to the one it approached on, coming back to the same perigee--or actually a bit lower, so it would reenter the atmosphere.
If SpaceX is not going to provide a souped-up super-Dragon with lots of extra propellant for heavy delta-V near the Moon on the nominal mission, but plans a simple flyby, they need to send the Dragon on an extra-energetic transfer orbit similar to those used by Apollo. Thus the TLI speed near Earth is slightly higher than for a Hohmann orbit. If they were planning to use a Hohmann orbit it must be that the Dragon does need have thousands of meters/sec delta V available. I conclude they are doing a free-return flyby.
In that case, the apogee of the planned transfer orbit is far beyond the Moon's orbit, and while any slowing of the speed immediately after TLI will lower that apogee, it has to be lowered a lot for the return to be sooner than the planned loop around the moon. Otherwise they fall between stools, missing the Moon because the timing is off, flying far out into translunar space--not as far as they would without the braking burn, but then they'd have encountered the Moon as planned. And since the Moon interrupts their outward orbit halfway and sends them back down going fast, they'd do better to just let it ride, unless they had so much delta-V they could lose a lot of speed and get to an orbit with apogee far closer to Earth than the Moon's orbit.