A new client has signed a contract to say that they would take X for £X. We have a clear paper trail and we have always been very transparent about what the price included.
Now the first invoice has gone out, they are saying that, despite what the contract says, they thought it was half. They raised a purchase order for less than the full amount (which admittedly wasn't double checked by my accounts lady) and have said that is all they will pay for.
I have written them a reasonable email copying all the key documents back to them, and saying that, under the circs, the mistake is at their end and that they should pay what they agreed. They are now playing merry hell.
It's an influential client, so I don't want to piss them off, but on the other hand I feel as though they're using the might to pull a trick on a much smaller business. WWYD?
ETA verbal negotiations were done my an employee, so I cannot be 100% sure of what she said, although I believe her to be reliable and trustworthy.