I've managed timberlands on an industrial scale (100,000 acres to 1,000,000 acres) during my career.
I suspect Roseburg doesn't see your five acre offer as worthwhile. Large land owners are reluctant to "carve out" little pieces here and there because they often become an encumbrance to their typical activities of harvesting, burning, spraying pesticides, leasing hunting rights, etc.
I can't tell you how many times we had little pieces where sometime in the past an acre or two... or ten... were sold to somebody to build a cabin or whatever in the middle of a remote forest. Over time, these owners exert their ownership rights with the county judge, courts, etc. and demand their larger neighbor (the timberland owner) who sold them their lot cease their traditional timber activities because it interferes with their enjoyment of their small acreage.
Not criticizing your desire to secure the five acres. In fact, good luck with it.
Just offering perspective for why the large landowner might be lukewarm to it.
Perhaps if you offered to sign a waiver against future claims, but I'm not sure those are worth the paper they're written on.
Edit to add that leasing should be more attractive to them then selling the land. A lease should give them comfort they could always cancel if you became in their eyes an "obnoxious tenant."