Bought a bed on sale but now I have to pay extra

I bought a bed at morkels on the 10th of January 2015,the restonic bed's original price was R3999.99 however upon paying we were told the bed was on special R3499.99 which we paid in full over a month later the bed has not been delivered because they are out of stock and we had to pay R500 extra to get the another restonic bed delivered as it is no longer on special.but a bought it when it was on special and i want my bed,is it my fault that the bed is no longer in stock or manufactured.DISSAPOINTED