As long as the wire under the insulation isn't exposed, believe you're all right. Use some tape and repair the damage on the case and see how it reacts to charging. What you're watching for is to see if the lipo swells up when charged.

If it starts to swell, stop charging immed and take the lipo somewhere where it can't cause any damage and leave it for several hours.

My guess is you're going to be ok, just cracked the case. You need to use more ways to restrain the lipo better in your Slash - try using a piece of self adhesive velcro in the tray and on the lipo so it can't move. I use a 1" wide piece the length of the lipo and mine simply don't move under a hard hit preventing any lipo damage.