Please help me with this

Facts: Dusky Bat has decided to purchase a new house. He is a keen animal lover and finds a house with a large pond. This pond contains several varieties of frogs. Before he puts an offer in to purchase the house, he states to the current owner, Adi Rompo: "Your pond and frog life is amazing, I really love the idea of buying a house with a pond and frogs, so that I can hear the noise of frogs every day." Dusky puts an offer in to purchase the house. It makes no reference to the pond or to frogs. A plan of the house and land is attached to the sale contract and this does show the pond marked on the plan. The offer is accepted, and a contract formed. On the day of settlement, that is the day that Dusky is to pay for the house he does a pre-settlement inspection. When he attends the property, he is dismayed to see that the pond has been removed and the hole that was there previously has been filled with dirt. Dusky contacts Adi Rompo and states that he will not settle. He states that he does not wish to complete the purchase as he was only going to move in if there was a pond with frogs. Adi Rompo says that was not a term in the contract. He says that as a goodwill gesture he will reduce the sale price by $5000. He says that is more than enough to put in another pond. Dusky knows that it takes years for frogs to grow in the numbers that were in the pond and that in fact frogs may never move back into a newly established pond. Dusky believes he is entitled to walk away from the purchase contract. The seller believes he cannot. Required: Please advise both Dusky and Adi. Please consider the legal principles of the importance of terms in a contract, collateral contracts, the parol evidence rule and any other legal principles that you believe are applicable. Please refer to case law and legislation as you think appropriate