Change in MRP of goods

I have ordered water heaters in advance last year at an agreed dealer price of 4500 per piece (MRP 5000)… however at the time of delivery now the company has sent me goods printed with old MRP of Rs 4800… now the company is saying that they will paste an additional sticker with new MRP 5000 above the old MRP printed.. can it be done and what are the legal aspects.