First prepayment, now partial payment demanded - confused!

Regarding the invoice: 50% was the down payment, now he wants another 30% converted - 20% remain

30% is justified if everything is ready for you to move in. For defect correction, a retention of 5-10% is typical. A doorbell and a few crooked sockets do not justify a 50% retention.

so a personal conversation (or phone call) is surely more sensible than immediately swinging the letter or even the lawyer’s hammer. as musketier already wrote, 20% retention is completely fine if basically everything has been installed and only a few minor things still need to be done. so call the electrician and clarify with him when he wants to finish the rest and then probably pay the 30% (and also say on the phone that you will do it). a few crooked sockets and a missing switch are not a big deal and depending on building regulations not a problem (missing wiring before plastering is really no issue) until something went wrong (wallpapering with missing wiring). straightening a socket or connecting something is really no problem.

There are still other things missing: motion detectors, satellite system, LED outdoor lighting...

What matters to me here is the way: we agreed in writing on 50% sales price and the rest after completion. I informed him of this in writing. Then to come and want an installment payment, although much has not yet been delivered, and also in that tone, I find that inappropriate. But I will transfer it now and withhold the rest until completion.

If you believe that the remaining 20% amount is not sufficient, simply deduct 5% or 10% retention from the amount to be paid so far (advance payment + installment) and write this as well in the transfer "installment invoice less x% retention". This is quite common in construction if there is no completion or warranty bond.

Apart from your communication and the probably well-described questionable manner of the craftsman: The house has neither been completed nor has an acceptance taken place yet, right?! Can you even speak of a defect if you have to assume that the craftsman is not finished at all? As I understood it, you are building with a general contractor, and extras have been commissioned from the electrician on the side... What reason did he have to be gruff on the phone? So, in #1 it says something about "not finished yet", later you have already moved in?

Can one even speak of a defect if one has to assume that the craftsman is not yet finished at all? The service provided may be defective. This condition may entitle one to withhold payment for the service provided and also to refrain from making further payments until the defect is remedied.