all this started because the managing agent requested an amount for a room to be re-painted. by your own accord they were good tenants, pay the amount for the painting of the room and negotiate for them to drop their claims. the managing agent is acting on your behalf but surely they should inform you of any requests ??? if the managing agent came to you first would you have agreed to pay for the room ? if so then maybe request all claims be run by you first to stop this from happening in future.
They have no claim... so why should he negotiate?