Refuse to pay full deposit refund

Had a contract with them for over a year and didn't want to renew a contract for another year. I request that I be moved to a 2bedroom to a 1bedroom for only 3months as I will be moving out end of December 2017. On the 3 Dec 2017 I sent a reminder for notice of termination. That I will be moving out. And my email was acknowledged. On the 2nd of February 2018 I received my R2300 deposit refund instead of R6300. And was told that due to late notice of termination of lease they deducted R2500.