Actually no.
These civilians have no diplomatic relations and are not being told by their government to be there.
so these people are gonna die.
Countries don't have the right to kill citizens of the European Nations unless they are violating a law and would normally be killed as a citizen of that country. These people who are marching are not criminals, and will not be violating any laws of Syria unless marching across a desert is worthy of a death sentence. If Russia decides to do an air strike they will be held accountable for war crimes as well as violation of international law with Germany.
If they fail to yield to the civilians, Germany will probably forget Russia's trade over or become an enemy/hold them accountable by force. This is how real international politics work. However, if the United States-backed rebels attack the civilians, Germany will only be able to hold the rebels accountable, since it isn't direct involvement. They might be able to forget the US for supporting them though.
Think back to the Nanjing Atrocities. Since Japan was at war with China, they were able to attack whichever Chinese civilians they wanted. However, the moment European citizens intervened and formed a safe zone, Japan couldn't actually touch them. Likewise, civilians, even with no diplomatic relations, can still apply for a special status that makes them humanitarian workers, and as a result, Russia won't have the ability to touch them without having severe backlash.