What is the ruling on investing other people's money for a certain percentage in exchange for bearing 70% of the loss and splitting the profits equally, or by paying a fixed monthly amount for a specified period then retrieving the capital with profit, and what is the legitimate method for murabaha and office fees for buying and selling shares for others?

Your question comprises three issues:

1. Your investment of others' money such that they receive 50% of the profit, while you bear 70% of the loss: This contract is impermissible because the mudarib (investor) does not bear a financial loss. Its rectification is achieved by agreeing upon a specific percentage of the profit, with the mudarib's loss being his effort and the capital owner's loss being from his capital.

2. Your investment of others' money with the condition that you give them a specific amount: If the monthly amounts are loans to be repaid from the profits with the differences settled, then there is no objection to this transaction, provided that the capital is not guaranteed. If the monthly amounts represent the profit or a portion of it, then this transaction is impermissible because the profit in mudarabah (profit-sharing investment) cannot be a fixed amount. This constitutes riba (usury) and a guarantee of capital, which invalidates the mudarabah contract.

3. The manner of buying and selling shares: There is no objection to this for you if you adhere to the Shariah conditions and regulations.