I've just found that, 19 inches OEM wheels have different Rim size, I just installed a set of tires, and was told by the Strass Auto, that the rims are indentical. (Bull)

Now, my left rear wheel is installed with 245/40/19 on the rim size 8', meaning they switched the rims but putting the correct tire size.

Can this cause any problems? or must I bring the car back?

yeah butter get that fixed!

isnt there a bolt so the fronts will not fit on the rear?

Ya, but I removed it when I installed my winter wheels. Weird that the rim itself doesn't specified any sizing. All it says is "Forged"