In the Netherlands, am I obligated to refund a previous employer for missing deductions?

Question

The situation is next one. My previous employer made a mistake and instead of deducting 300 EUR a month for my train subscription, they were paying me it extra.

I treated this amount as their compensation for traveling and several times contacted HR that I don't see a deduction.

I left this company and finally they found the mistake. Right now they are asking me to pay around 5000 EUR back. I can not and don't want to pay this amount in one time. I tried to negotiate it to pay in smaller amounts. They are fine that I pay it partially but asking to pay in bigger chunks.

Today I got a letter starting with "Politely I ask your attention for the following..." and it comes from their lawyer. I assume it might come to the court.

I have a small hope that someone could give me thoughts about my rights. I also applied for legal insurance but it will start only in 3 months.

Answer 1

This is a very partial answer, because while I live in the Netherlands, all of my labor law experience is knowledge is from a different state and legal culture (Israel). Still, it might be useful to you.

1. You must at least reply. Regardless of whether you reach the conclusion that you owe them the money, or reach the conclusion that you don't owe it (or owe only some of it), you must reply to their letter within a reasonable amount of time and state (perhaps even explain) your position.
2. Don't assume you were "paid extra". If I understood correctly, it's not that you got +300 EUR for a train subscription and the train subscription itself, but rather that you got the subscription but they did not make a corresponding deduction. It is not at all clear that the situation is that you were "paid extra" - and do not concede that this is the case even if they tell you it is. You have to scrutinize your written contract and its exact wording on this matter; and whatever collective agreements are in effect (see also my next point).
3. Consult the relevant union. In the Netherlands, most employees are represented by a union in some way or another. Sometimes your union rep is someone down the hall, sometime you just hear about union negotiations with management on the national level. At any rate, try to figure out which union you were represented by, and go talk to them. (Doing this online is not always useful, it would probably be best to do it in person and with someone who speaks Dutch.)
4. The custom may trump the contract, legally - it's possible that even if originally your employment contract mandated a deduction, your employees non-deduction cannot be legally undone after the passage of this much time. This depends on Dutch law, especially labor law: Is the employer considered to have been aware of what it was doing, after enough time has passed? Which party is responsible for an employer's negligence? Is there legal significance to the fact that you had made expenses based on the knowledge that your salary is X rather than X - 300? It's possible (but not certain!) that the law in this case might mandate you not having to return the money, or only having to return upto a certain amount, or upto a certain time into the past etc.
5. If you are actually threatened with a lawsuit, consult a lawyer. 5,000 EUR is enough money to merit at least a one-time consultation with a lawyer. And it is not unlikely that the relevant union could help you with this, either with free legal consultation, free representation vis-a-vis the employer, or suggestions regarding lawyers to contact.
6. Even if somehow you end up acknowledging you owe them, insist on a gradual refund. I would almost bet that even if you owe them the money, they cannot legally expect it to be paid immediately. So at worst you can at least insist, I would think, on paying back not significantly more than a certain fraction of your current wage. Again - I can't be certain, it's just my legal intuition.

Answer 2

You worked for the company and received payment. You thought you were paid well, that's why you stayed with the company for 16 months. If your pay had been lower, you might have looked for a job elsewhere. Now they claim that your pay was actually €5,000 less than you believed.

That would be greatly to your disadvantage. Even if they were right, by paying you more than your actual salary all the time, they made you believe you were paid well and made you not look for better paying jobs. And you didn't put that money into savings, you spent it because you relied on them getting your salary right. So you have some excellent arguments why you shouldn't have to return that money, because if you had to pay it back, their incompetence would be to your huge disadvantage.

Talk to a lawyer.