Here's a simpler version of that link; hope that works.
http://laws.justice.gc.ca/en/showdoc/cs/c-52/bo-ga
I think the difference is that in the case of debt; one part of the contract has already begun. Let's say I walk into Ford's store and ask you "Can I take this widget home today, and I will pay you tomorrow." You agree and I owe you $100. Tomorrow you refuse the $100 payment, so the debt is wiped out; as I tried to hold up my end of the contract but you refused.
In a normal retail environment, we are still negotiating the contract when I offer to buy the widget and give you the $100 bill for it. Because we have not agreed to the contract yet, you are under no obligation to enter into that contract.
But what is really happening now is retailers are either refusing old $100 bills, or refusing to make change for $100 on a $5 purchase. Both are legal in any circumstance; because technically once a new bill is issued by the Bank of Canada, the old bills are supposed to be surrendered back to the Bank of Canada for replacement. So technically they are no longer legal tender. In the case of the change for $100- the merchant is refusing to enter into a contract with you that will prevent him from making any further sales; due to you having all of his change.