In the scene when Melanie steals out of her mother's purse, the dollar bills are American. The show takes place in Canada.
Melanie's mother is a struggling, single mom who is having trouble keeping money just to pay for groceries. However, the fact she only seemed to have American currency in her wallet was highly unlikely for a Canadian in her case. In 1989, ATMs were brand new and everyone still did banking inside banks with a teller to make deposits and withdrawals. Melanie's mother would have very likely been working in Toronto, too far from the US border to be paid in US cash. To have US currency in one's wallet usually was almost always reserved for saving for trips across the border or if someone saved the American money to exchange later when the Canadian dollar went down. To have it in one's wallet for everyday spending was just not something Canadians did. Because of the value, someone like Melanie's mother would have most certainly exchanged the cash for Canadian tender. Even teenagers were well-aware that $20US was more valuable, and Melanie wouldn't have outright stolen so much money knowing it was worth more in Canadian dollars. It would have made sense to have some Canadian currency in the wallet, or to at least acknowledge that the money was meant to be exchanged. For this reason the prop of American currency was deliberately used to appease American viewers to see what is familiar for cash.