Recording a loss

No, you don't need to do anything special.

You purchase it, enter it in your accounts and it will appear in your figures as an expense. That's all you need to do.

What you could do for yourself (and for clarity in case of a tax audit) is note in Dexxter that it was for customer XYZ and that it was a free replacement. You don't have to, of course, but it might be useful.

Let me know if you have any questions, I've been in your situation before ;)

Can I also issue an invoice for 0 euro for the customer? Because I honestly have no idea where to note this in Dexxter.