I think I figured out why I thought this was an issue...I am overthinking it. Unlike PayPal, where you go to paypal.com and log into your account to send money...with Square Cash you just send an email.
Go to your regular email account (you do not need to go to SQUARE), and send an email to someone (me, for example). Address it to my regular email address, like marcissmart@yahoo.com and in the CC: field, put cash@square.com. In the subject field, put in the amount of money you are sending, like $400.00, and in the body of the email you can put anything you want. Like... Marc, here is the $400 I owe you for making me such a beautiful pen.
Hit send and I think you will get a email from SQUARE asking you what debit card you want the funds to come from. Then on my end, I get an email that says you sent me money and what debit card do I want it credited to.
That easy.
Actually, even easier, because the second time you send or receive, it will automatically use the debit card you selected the last time.
Or, you can download the app for iPhone or Android and it is easier still after that.
When you receive money, you do not have to do anything. It goes directly to your debit card for credit (shows on your bank statement as a debit card return for credit). The first time it takes 2 days or so to show in your account, but after that it is really fast. Like next day.
There are no fees on either end.
I do not know about Canada, sorry. Other than it is cold there.
HTH.