Brass bolsters stored in a sheath will definitely promote verdigris. If left long enough, the verdigris will actually etch the brass.
A while back, I bought some Vapor Phase Corrosion Inhibiting plastic zipper bags. VCPI or even VCI for short. They are available in many sizes. They are used for long term storage of metal parts and pieces. The ones I bought are sized such that most of my folding hunters with brass bolsters (Camillus, Buck, Schrade, Case Shark Series, etc.) can be dropped inside, zipped closed, and THEN put in the sheath. The bags provide an effective barrier between the leather and the knife. And, the VPCI properties of the bag's interior provides added corrosion protection for the knife in general. When purchased in bulk, the cost of the bags can be as low as $0.03 - $0.05 each. I went in with several other collectors and we bought a case to bring the cost down. I also obtained some larger ones that are great for items like handguns.
They also make VCPI/VCI paper too. In fact, that is what Bear & Son wraps their knives in from the factory.
"Now it cuts like a knife, but it feels so right." Bryan Adams