Subsidies may distort the market but they don’t change the existence of the supply/demand curve. Any producer of a product is going to sell their goods to the highest bidder, and if someone is capping what a product can sell for that means capping what they can purchase the product for. Grocery stores aren’t going to sell for a loss.
If the central government enacts a scheme of rationing and central purchasing, that’s one thing. But in a free market situation if a grocery in country A will buy lentils for €1 a kg and country B can only pay €.50 then the lentils are getting sold to country A until that demand is fulfilled. Which means shortages in country B.