The rental deposit does not serve to cover claims from additional costs.
With the handover/return of the apartment without defects, the claim to the rental deposit is forfeited, as all regular claims (the rent) have been paid.
=> Request in writing the repayment of the rental deposit, set a fair deadline of 4 weeks. Afterwards, write a reminder with the note that the matter, along with further costs, will be passed on to a lawyer if the money is not received 2 weeks later.
Then include this as well and it's done.
"The landlord may retain a [B]reasonable part of the rental deposit after the termination of the tenancy if a demand for additional payment is expected from the operating costs statement that has yet to be prepared regarding advance payments made for this (AG Gießen: judgment of 10.05.2012, 48 C 352/11)."[/B] mietrecht.org
Given the disclosure of the address and no outstanding payments (both rent and additional payments), I even see only an amount of 0.00 euros as reasonable. As long as the additional costs have been sufficient in the past anyway, there is no expectation of a demand for additional payment.
And if all the niceties and kindness are not enough, I recommend legal advice.