Is it permissible for the seller to demand an amount exceeding the agreed-upon price of the commodity three years after the full price has been paid, under the pretext of rising prices?

The transaction mentioned in the question falls under the category of manufacturing contracts (istisna'), where the manufacturer is obligated to provide both materials and labor. Istisna' differs from 'ijara (leasing/hiring) because the manufacturer provides the materials, and it differs from salam (forward sale) because it requires labor. Scholars have differed regarding the ruling on istisna' and its conditions. The Islamic Fiqh Academy has stipulated that the timeframe (ajal) must be specified in such contracts. According to the view that requires the specification of the timeframe, the aforementioned contract would be void and should be annulled. The questioner should claim back the price from the manufacturer, or enter into a new contract with him. If the questioner does not agree to the increase (in price), he is only entitled to his original capital.