Your partner uses a ute for his business. But after it broke down, his family offered to buy a new one. You got one at a great price which they kindly paid for, and arranged to have the tray modified so it would better suit your purposes. The family are unhappy about it and prefer it to be kept in original condition. You don't want to cause any ill feelings with the family, so how can you explain why it's imperative for you to do this, without a fight?