What is the ruling on purchasing a pharmacy with financing from a partner, with profit sharing for a limited period with a minimum, and an unlimited period with no maximum, provided that the capital is returned at the end of the contract?

If the partner fully funds you to purchase a pharmacy and work in it for half the profit, this is a permissible Mudarabah (definition) (partnership). Specifying a minimum duration is permissible according to some scholars. However, requiring the financier's capital to be returned at the end of the Mudarabah such that the financier bears no loss is not permissible.