Can't transfer to self-custody wallet of another person

Hi all,

I (living in Germany) am trying to transfer Bitcoin to another person in another country as a payment (he requested it this way).

He uses a self-custody wallet, so it's not at any exchange or broker, he says it was downloaded at bitcoin.org.

I've aquired the amount of Bitcoin needed but when I try to add his wallet self-custody in the broker UI (I've tried Bitpanda and Bitvavo), I have to confirm it is my own wallet (which would be a lie because clearly it's his wallet).

What am I doing wrong? Would appreciate some tips.

Thanks in advance <3