This amount of money is actually possible for a player with one year left on his contract, but it's unwise to pay that much for a goalkeeper. Therefore, if Paris Saint-Germain wants to sell their goalkeeper, they have to consider the price. No team would pay him that much with his contract expiring. While he's a very important player for Manchester City, City could sign him for free after a year. I'm sure they've held discussions regarding the player. If the Italian goalkeeper wants to join Manchester City, he can stay at Paris Saint-Germain for another year.
€25 million euros seems like a fair price. No matter how good Donnarumma is, no club would pay the amount PSG is asking for a goalkeeper that has got just one year left on his contract. Also, PSG should know they don't have the upper hand. If they don't sell him this summer, they would still pay his wages and lose him for free next summer. However, if they sell him right now, they would get some money back, which can be reinvested in the squad.