Yes, bill of sale. Google a generic one and fill it out, take two copies one for him and one for you. That should suffice for temporary tags (or just keep it in the vehicle if Wisconsin is as easy going as Iowa).
Bought many cars that way, never had a problem, but best bet is to call your county registration group and ask them to be safe.
Herbert - '12 X5 35d / Sapphire Black / Black Perf Nevada / Aluminum trim / ZPP / ZPS / ZCW / ZAV / Apps / 3rd row
Betty - '04 CTS-V / Black Raven / Black Leather & Suede / 6 speed LS6 combo 400 hp & 400 tq
Sterling - '06 Silverado HD / White / Grey Leather / LBZ Duramax - Allison 6 speed combo / Crew Cab Short Bed LT3