Do I have a right to have them confirm the repayment to me?
My fear is, on the one hand, that their bank account number has
changed.
This is very unlikely.
But, this fear is unfounded and unreasonable.
The practical reality is that if they received a letter explaining that you are sending them money and they didn't receive it, they will promptly let you know.
If the account didn't exist or had a different owner, the bank probably wouldn't even process the charge and would have sent you a failure to transfer funds notice.
Normally, your business and banking records would suffice to prove that you returned the deposit and the basis upon which you made your deductions.
You should save these records, at least, until the statute of limitations for disputing the security deposit deduction has expired (unless you returned it to them in cash, which you should never do and isn't part of the fact pattern in the question, except in person with a receipt signed contemporaneously for the cash).
I have a feeling that the tenants are not happy with me deducting
repair costs, and with confirmation of the refund, I could close the
whole story for myself.
You are confounding an acknowledgement of receipt of payment, on one hand, and on the other, a release of any liability you have to them (or alternatively, an affirmative statement that there are no present claims outstanding from them against you which when made before the end of a lease is called an attornment in English - I have no idea what it would be called in German).
A mere acknowledgement from the tenant that the funds were received would add nothing to what can be proved from these business records.
You could always ask them to sign a receipt and release, and sometimes people will sign it, because people will sign all sorts of things that are not in their interest to sign to make it easier for them to get money paid to them. (I don't know if that kind of receipt and release would be valid in a residential lease in Germany, but most people would assume that it is and refrain from suing over the deductions even if they could win in court.)
But, ultimately you would probably be breaching the lease and be vulnerable to being sued by them if you refused to return the security deposit because they didn't sign it.
Your question suggests that what you really want to do is to condition your repayment of the undisputed portion of the security deposit upon a release of further liability on your part from them in connection with your deductions from the security deposit which you don't know if they will dispute or not.
But you very likely aren't legally permitted to withhold security deposit funds that you don't dispute that you owe them for this reason.
This is true even though you sending them the money is strong evidence (although probable not a conclusive legal waiver of rights on your part) that the money returned is not in dispute and that you have no other claims against the tenant of which you are aware at the time.
If they continue not to respond, could I instruct my bank to reverse
the transfer?
No. You paid them money that you admit that you owe them. Reversing the charges might even be some kind of fraud or theft on your part.
If so, how could I enforce it?
It would also never make economic sense to sue to enforce a provision requiring an acknowledgement from them in a lease, or seeking declaratory judgment that your charges for the non-returned portion of the security deposit are valid.
It might be possible to write a provision in your lease that imposes a shorter than the usual statute of limitations time period (e.g. two months from receipt of your funds and an explanation of them) for them to dispute the charges, and providing that if they don't that their rights to dispute the charges are contractually waived.
Then, if the tenants sued after this deadline to dispute the charges, your defense to their claim would be that they didn't raise the issue with you within the deadline set forth in the lease.
I don't know if there are particular German laws (possibly only applicable in certain states or cities) that prohibit this kind of term or not. Germany, generally speaking, is fairly protective of tenant rights, especially in major cities, so I would not be comfortable recommending that you include a term like that in a residential lease without you confirming its validity with a German lawyer or notary who practices in the particular city where your apartment is located.
