Wis. Stat. § 703.36

Any provision of a declaration or other instrument made pursuant to this chapter which requires the owner of a unit to engage or employ the declarant or any subsidiary or affiliate of the declarant for the purpose of effecting a sale or lease of any unit is void. Any provision of any contract for a sale of any unit which requires a purchaser to engage or employ the vendor or any subsidiary or affiliate of the vendor for the purpose of effecting a sale or lease of any unit is void. This section applies to declarations, instruments and contracts made prior to and after August 1, 1978.