Praise be to Allah.
So long as you know that you did not withdraw more than 3500 riyals, then you should pay them this amount and no more, if you can do that.
If you cannot avoid it – such as if they threaten you with jail etc – then negotiate with them to reduce the extra amount rest as much as you can. You also have to repent greatly from this haraam transaction. And Allaah knows best.