The WIFI wasn't worth a darn and we paid for their...

The WIFI wasn't worth a darn and we paid for their fastest speed. I cancelled my service when my year agreement was up. They charged me $100 to cover for the $13 balance owed. Now I'm being told that I won't be credited back my $87 credit for 60 to 90 days. Essentially they're holding my $87 for 3-4 months for now reason.