Problems with subcontractor's invoice

The subcontractor says that was not part of his contract with the general contractor

And you rightly say that you do not have a contract with the subcontractor.

==> Problem of the subcontractor to obtain his payment from the insolvent main contractor or the insolvency administrator

If the main contractor submitted a supplementary claim before the execution, then the project owner must also pay the main contractor, but not the subcontractor.

Everything legally relevant has been said, but I wanted to briefly bring in another perspective:

Is the subcontractor for the electrical work or something else? Are all the works that fall into this area completed, or are there still further works that you will have to commission yourselves? I ask because, for example, with us, the rough-in electrical installation was completed before the insolvency of our developer, but the basement installation and fine installation were missing. We would have preferred to continue working with the old electrician, but he didn’t want to, and the new one swore a lot at times (partly because everything wasn’t labeled very well, so everything had to be traced through; partly also special quirks that the previous electrician had come up with, for which of course there was zero documentation; etc. etc.). I don’t know whether our electrician sometimes made things a bit difficult, but we were also advised on all sides beforehand: If possible, continue with the old one. (He just didn’t want to...)

Otherwise: that wasn’t the case with the electrician, but other subs also wanted to have invoices paid by us first. We had to reject that as well, but partially communicated a willingness to compromise (“we’ll find a solution”) especially with trades that might only have been half completed. I don’t know how much everything has been completed with you, but I just wanted to put that out there at least.

I just reread the initial post. So the electrician continued working after the insolvency of the GÜ? And he wasn’t commissioned by you for that, but he just did it on his own? How did that come about?

has now sent an invoice for work that was allegedly not commissioned with him.

The general contractor paid him in full:
The subcontractor says that was not part of his contract with the general contractor

Ah, now I understand. According to the general contractor, the electrician allegedly carried out additional work that is neither listed in the scope of work description nor recorded in the contract.
I would check that if I were you: it’s not uncommon for the client to make an oral agreement with the subs on site when discussing details and implementation. If on site you commission a glowing light switch, a changeover switch, an additional fuse, an outdoor cable, or one more socket, then that has to be paid for as well. I don’t know anyone who hasn’t made an additional order, especially in electrical work, with their subcontractor. Because especially in electrical work a lot is assumed by the client that is not included in the turnkey offer.
What exactly is it about?

I would check that in your place: it is not uncommon for the client on the construction site to enter into an oral contract with the subcontractors when discussing details and implementation with them

That was also my thought. If you have given oral directions, it is only fair to pay for them as well. What surprises me: How do you know that the subcontractor was fully paid by the general contractor? You have nothing to do with this contractual relationship.