How to deparent object between frames in Maya?
Use constraints to do this:


Select the parent (the hand), then shift + select the child (the sword). Warning, selection order matters.
Rigging > Constraint > Parent Constraint > activate Maintain Offset if there's an offset between both pivot points.
Set a translation keyframe (shit + w) and a rotation keyframe (shift + e) on the sword.
By setting those keyframes, the sword now has a new attribute in the Channel Box, called Blend Parent 1. You can animate it to choose between unconstrained (value = 0) or constrained (value = 1) by the hand. If unconstrained, you have to set keyframes on the sword to place it inside the worldspace.