Can my employer withhold my last paycheck, to offset a partial bonus I received? I live in WI.

I was hired in August of 2015, and signed an agreement, stating I would receive a signing bonus, to be paid out in three parts.

The first part was paid out with my first paycheck, the next part will be paid after one year, and the third on my second year anniversary.

I have just put in my two week's notice, and am wondering if they will now be able to withhold my last paycheck for repayment of the partial bonus.

The agreement states:
"A repayment must be made, or an acceptable repayment plan approved by the organization must be finalized prior to my date of termination of employment or date when my availability for employment becomes less than FTE level that existed at the time of hire."

I am also worried that my employer will retaliate, and terminate my employment, as a result of my submission of the letter of resignation. Is this legal?