Well they cannot possibly ask for 10.000 additional Euros, that’s the whole point of buying off-plan, the value of the property goes up. It would be illegal. I’m surprised your lawyer hasn’t mentioned this.
Problem is you failed the payment on the specified deadline. That’s serious. The contract normally has clauses regarding this case. Can you get your bank to put down on writing it was their mistake ?
I would procede to pay the remaining amount ASAP by means of a “consignación judicial” If necessary. You must pay immediately the remaining amount despite them refusing to accept it. You place the money in a specific account of a Court suitable for this purpose so there is no doubt you’ve paid and fulfilled your duties. The longer you delay this you will be breaching the contract and they can resolve the contract and keep a % of your money as per stated in the contract’s clauses.
I dislike them and their shabby tactics. I would avoid them at all costs. Their owners are now in prisión prevntiva IIRC and their personal assets have been seized/frozen by a court ruling.