In what year to deduct tax consultant expenses?

I'm currently making the tax return for 2019.
In 2018 hired a tax advisor for the 2017 declaration.
I got an invoice for that.
The invoice was issued at the end of 2018, but was not paid until the second half of 2019.
Would I have had to deduct the expenses in 2018 or can I deduct them in 2019 because the bill was only paid in 2019.
How do I determine the amount that I can deduct for a notebook that I have used for my studies and also privately?

In principle, invoices are always issued according to the payment date. If you paid in 2019, this also applies to the tax return.

The date of the service provision or the invoicing does not matter.

Depends on the type of income or the type of profit determination. But I assume you don't make a balance sheet?

Then the inflow / outflow principle applies. Expenditures must be taken into account when they are paid