Tenant Rights Apply - Wisconsin Specific

[Casandra M.]

We have an accepted offer on a place that was occupied in 1 unit, the other unit empty. After the accepted offer, before closing, the current owner signed a new 1-year lease through October 2019 at an under-market rate to a new tenant, which completely changes the expected cashflow of the place for us as the upper unit is also under-rented. 

Are there any legal issues with the current owner signing new tenants after they accepted our offer? I understand that tenant rights apply when an offer is accepted... what about during the transition period?