Deposit Dispute

Tenancy Deposits

My tenant moved out in the 4th month of her 6month tenancy agreement.She held on to the keys and I had to promise her she would be paid her deposit backif she released the keys.
I then realised all bills were unpaid and i decided to dispute her claim to the deposit on the basis of the fact that she broke the contracts by moving out before the end of her tenancy. She is claiming that I promised to give her back her deposit using the text I sent to promise her the deposit would be refunded. Who is entitled to the deposit under the law.