Unfortunately, the rear left seatbelt got stuck and can't extend.
Brought it to the dealership and was told that the only solution is replacing it with an OEM part which comes at a cost of around 1000 euros.
Hello and welcome to the forum!
Question: Did the seat belt in question got stuck after you bought the car? - If no, you should talk to the seller!
A 1000 € for a single seat belt replacement sounds quite steep to me!
I presume that your car no longer is under any kind of warranty (otherwise, incidents like this should be covered by it).
In your position I
most definitely would seek a second opinion from a different Honda dealership!
If told the same figure I'd look into getting a "new" seat belt from a scrap yard.