Thanks for the great question Phil!
Call me old fashioned, but payment should be due in full prior to your lumber guy taking your logs. What happens if the lumber guy says the logs did not yield the quantity and quality of lumber he thought? Unless you had a signed contract stating such and such dollar per board foot, then you do not have much to stand on from a legal standpoint. Bypass this potential conflict by agreeing to a price per board foot (in writing) prior to the logs leaving your yard. Treat this whole process like a business transaction. If your lumber guy balks, take your business elsewhere.
Based on data from my TimberBlog, the black walnut market is strong: board foot price for sawlogs is roughly $1.00-3.00 and $2.50-12.00 for veneer logs.
Hope this helps!