Well from what i have seen Samehada does not automatically absorb chakra when ever a person touchs it (aka gai touched it and dident get his chakra sucked out)
Only time it seems to absorb chakra by touching is when They have a visible ckakra cloak like Bee/naruto had or absorbing it from jutsu's
It even says on naruto wiki
"However, it seems to be unable to automatically absorb ambient chakra, in all instances where it's absorbed chakra are when his opponent had a visible aura around them"
So minato can touch Samehada
Only time it seems to absorb chakra by touching is when They have a visible ckakra cloak like Bee/naruto had or absorbing it from jutsu's
It even says on naruto wiki
"However, it seems to be unable to automatically absorb ambient chakra, in all instances where it's absorbed chakra are when his opponent had a visible aura around them"
So minato can touch Samehada