Most knives and multi-tools are forbidden on commercial airlines. There are very specific items offered from a few notable manufacturers that are designed to be tools that are TSA approved.
Consult with the manufacturer's warranty to see if such damages are covered. If so, you must go through the manufacturer directly for replacements or repairs.
Presently, there are no laws or regulations forbidding the carry of multi-tools within the U.S., provided it does not classify as a "switch-blade".
Most manufacturers do not cover standard wear-and-tear that can degrade the finish or coloring of the tool by it rubbing against scabbards, moving in and out of pockets, etc. You must verify with the manufacturer's own warranty policy to be certain if such damage and wear is covered.
Some manufacturers do offer options for replacement sheaths. We strive to carry as many of them as possible and you can navigate through all our options under the "Sheaths & Cases" found within our Knives/Tools category.