Canadian Business Law Case

Hi Please help me understand this case as I will be getting similar case in exam so need to understand the legal issues in this case

David and Hans were in a bar together and both had a few too many alcoholic beverages.They discussed David selling Hans his son's BMW (that he bought for him) because he was frustrated that his son had decided to do another year of high school rather than going to university.They created an agreement on a bar napkin and David and Hans signed with the bartender witnessing it. The agreement was that David would sell his son's 2008 BMW for $5000 ($2000 up front and the rest in $250 monthly instalments) and that Hans could pick it up in 2 weeks - giving David time to let his son know he would no longer have a car.In his drunken state, David had the year wrong and the car was actually a 2010 BMW.The next day, David had no recollection of the conversation with Hans.Two weeks later, Hans showed up with $2000 ready to take the car.An argument ensued and David threatened to call the police for trespassing if Hans did not get off his property.When David's 17 year old son discovered this situation he was furious and so David decided to go ahead with the contract because he did not like his son's attitude. David accepted the $2000 and gave Hans the car.David's son changed his mind about university and David needed some money for tuition so he went to Duncan and sold him the $3000 claim against Hans for $2500.Shortly after there was an issue with the car that was going to cost Hans over $5000 to fix so he stopped paying for the car. Duncan sued Hans and Hans then countersued David.

Explain the legal issues with each of these relationships and what you think the result of this situation would be if it went to court.