"I told him I can't send an invoice for USD'
Actually, it is possible to send an invoice in a different currency than the listing by using the PayPal "send an invoice" link.
It will not be linked directly to the eBay listings but will satisfy the buyer.
Most Americans prefer to pay in US$, the only currency they know and understand. That is why most Canadian sellers list on eBay.com or in US$ if listing on eBay.ca.