Accounting isn't always intuitive, but in this case I believe it is: His liability for the chicken is $25, because what he owes you is not "a chicken", but "$25 worth of chicken".
To see this, imagine what happens if he has to break his promise to deliver, perhaps because all his chickens get swept out to sea and there are no substitute chickens available in time. He owes you a refund. How much does he owe you? Having spent our lives doing deals like this, we intuitively know the answer: $25. He has to give back all the money you paid. That's why the liability is $25.
Now, once an actual chicken gets handed over to you, and you agree that it satisfies the chicken contract, things are different. Now the $25 liability changes into a $5 cost-of-goods-sold (assuming that wholesale chickens cost $5) and a $20 increase in equity (aka "profit").
To see this, imagine what happens if he has to break his promise to deliver, perhaps because all his chickens get swept out to sea and there are no substitute chickens available in time. He owes you a refund. How much does he owe you? Having spent our lives doing deals like this, we intuitively know the answer: $25. He has to give back all the money you paid. That's why the liability is $25.
Now, once an actual chicken gets handed over to you, and you agree that it satisfies the chicken contract, things are different. Now the $25 liability changes into a $5 cost-of-goods-sold (assuming that wholesale chickens cost $5) and a $20 increase in equity (aka "profit").
(More or less, I think. I'm not an accountant.)