Real Canadian Superstore

I was given 3 gift cards for $100 each from family. Went in and spent 2 hours with 3 kids doing a big shop for the month. Basically have rice and beans left in the house. When we went to pay, it turned out the gift cards sold to our family that HAD a superstore receipt and everything, were somehow fraudulent. They acted like we were thieves and said they had to put all our food back. Didn't apologize, offer to call the other store in the SAME city, or anything. I understand having to do an investigation of sorts, but that's their issue to work out later. Went to the other store they were sold at, and same thing. Didn't offer anything to rectify the situation. Now I have 3 hungry kids and my family is apparently out 300 dollars indefinitely? Not to mention I'm entering my second trimester and definitely dont need the extra stress or to be publicly humiliated. What an awful way to handle a known issue.