What's the chance we're looking at a buy-out with Nebraska? Here are the FCS teams with byes that match up with ISU and Nebraska.
10/20 BYES
Portland State
Montana
Eastern Washington
Gardner-Webb
Kennesaw State
Villanova
James Madison
Hampton
Savannah State
Florida A&M
Wagner
Austin Peay
Holy Cross
Colgate
Chattanooga
Nicholls
Prairie View A&M
Alabama State
Alabama A&M
10/27 BYES
Richmond
Elon
North Alabama
Bethune-Cookman
Duquesne
Tennessee State
Bucknell
Samford
Abilene Christian
The best case scenario I've found is to rearrange the schedule like this, buying out one existing FCS non-conference game. In this scenario no one would need to apply for a waiver for an extra game. Nebraska would be ok to replace an FBS team with FCS because they didn't have one already scheduled and they only have four conference home games, which should allow them an FCS game under Big 10 rules.
BEFORE
Sept. 29 Samford at Kennesaw State
Oct. 20 Iowa State bye, Kennesaw State bye
Oct. 27 Nebraska bye, Samford bye
AFTER
Sept. 29 Samford bye, Kennesaw State bye
Oct. 20 Kennesaw State at Iowa State
Oct. 27 Samford at Nebraska
The other possibility would be Sept. 22 Chattanooga vs. Samford which is non-conference game, but is one less week to rearrange schedules. Chattanooga also seems like a bigger buyout than Kennesaw St. with 20668 seats as opposed to KSU's 8318.
There a few other scenarios (and probably a few I missed), but they would interrupt an FCS conference game, which seems dicier, especially with the HBCUs where the conference championship may mean more than playoff consideration.
Sept. 15 Hampton vs. Tennessee State
Sept. 29 Savannah State vs. Bethune-Cookman
Nov. 17 Florida A&M vs. Bethune-Cookman
You should send this to JP