Freddie Mac mortgage servicers sending vacant property registration fees to the city of Chicago must do so under protest, according to new guidelines.

Each payment to the city must include a letter from the servicer noting the Federal Housing Finance Agency determines the registration fee does not apply to Freddie or its sister Fannie Mae.

The Chicago City Council passed the ordinance in July, requiring owners of vacant homes to register the property with the city and pay a $500 fee. The owners will also be forced to maintain the home while it sits empty. The city wants to cut down on the effect blight is having on the still struggling market.

According to Woodstock Institute, the area has more than 18,000 empty houses.

The FHFA challenged this rule, claiming Fannie and Freddie should be exempt in order to protect taxpayers from what it believes to be unnecessary losses. The two firms already owe $151 billion in bailouts. The case is still pending.

The government-sponsored enterprises reimburse servicers for this expense. Freddie Mac changed its guidelines Wednesday, forcing servicers to use a special set of codes and paperwork to receive reimbursement for any costs related to the Chicago ordinance.

After March 1, a servicer must be pre-approved through the reimbursement system for each expense, which could slow down an already delayed foreclosure and REO process.