Is it permissible to partner in a property without informing the other partner, and is that considered wrong, knowing that the other partner owns a smaller share in the property?

What you did by agreeing to build an apartment building in place of the shared shop without your partner's permission is a forbidden act, an injustice, and an aggression; because it is disposing of another's property without their permission, and this is forbidden as the Prophet, peace and blessings be upon him, said: "The property of a Muslim is not lawful except with his willing consent."

This action included demolishing the shop, which is destruction and aggression against another's property without their permission. If selling one's own share requires the partner's permission, then how about demolishing his property and using his land without his permission?

Therefore, you must repent to Allah and seek absolution from the injustice you inflicted upon your partner. You must compensate him for the demolished structure. He has the choice to approve or reject your action. In the latter case, you are obligated to cancel the contract with the businessman, unless you buy your partner's share with his consent.