You should have had a rate comfirmation stating 30k, if you didn't that was your first mistake. if a broker tells you or your friend or whoever 30k, make sure it says 30k on the ratesheet, if it doesn't write it in and send an email or call them mentioning you did so. You're in the communication business as well as the trucking business.
This is one of the benefits of hauling brokered freight, if the shipper wants to make a change then you politely tell him you need to make a phone call since your contract was for 30k. Call the broker and nicely tell him you need a new contract any you will be happy to haul whatever you can legally scale for "X" or else you will have to cancel the load and file a TONU. Let the broker handle the jerkoff shipper, it's his job. Try to work out something reasonable, but communicate with the broker. I usually ask if they'd like me to resolve the problem with the shipper, but usually they prefer to handle it themselves, and usually they are in contact with someone who may be the jerks boss, they have more leverage than you do.
If you handle these situations correctly you can often turn a lemon into lemonade for yourself.