Horst Grasz reached an agreement with Bert Allen Toyota, Inc., to purchase a 2003 Toyota Tacoma for $16,971, less a $1,000 rebate, plus taxes and fees. The sales manager entered the numbers into a computer and came back with a final price of $15,017.50. If the computations had been correct, the total would have been $17,017.50. Grasz wrote a check to cover the $500 deposit and the sales manager manually wrote on the sales agreement, "$14,517.50 due @ delivery." It wasn't until the truck arrived at the dealership approximately four weeks later that the sales manager discovered the error. The manager insisted Grasz pay an additional $2,000 over the agreed-on price to take delivery of the truck. Grasz filed suit. Bert Allen defended by arguing mutual mistake or, in the alternative, unilateral mistake. Who do you think prevailed? Why?