Buyer bought a computer pursuant to a final and written agreement signed by Buyer and Seller. The agreement states that the price is $2,000, payable on delivery. The agreement also guaranteed the computer to be free of serious defects. Prior to signing the written contract, the parties had orally agreed that payment was to be 30 days after delivery; and that the written contract was subsequently modified to change the price to $1,900. Can Buyer use parol evidence to prove any of the above? Why or why not?