generally in that situation I would explain this to them and hope they spring for a full eighth, or at least the dude could sell a half eighth. a lot of dudes around here sell by the gram for dank.
^^my buddy only wanted to spend a little...and that dude is weird. he sells via his mailbox, my buddy is pretty sure the dude doesn't even remember his face or name, and he's only ever gotten eighths for $50 my buddy is too nervous to change up the system. if it was me, i'd ask