Hang in there Marcel, this is common with all sheathes , and is a byproduct of moisture transfer between the leather and the organic scale materials. I have remedied such issues in the past with the following method:
1. Apply >70% isopropyl rubbing alcohol to the stained area (I like 90%, or you can use mineral spirits)
2. Using a coarse terry cloth rag, "scrub" the affected area, you will quickly notice the dye being removed from the scales and see it transferring to the cloth.
3. Repeat as necessary; once complete, wash the knife thoroughly and dry it.
4. If all stains are removed, apply mineral oil liberally, let sit, and wipe away
You should be good as new. I have done this on dye stained lighter woods, burls, and even white paper micarta and the results were excellent. And don't worry, Rick stands behind his work 100%; I'm sure he'll take good care of you either way.