Is it Islamically permissible for a relative to share in the profits at the stated percentage (25% of 75% of the profits) in exchange for his mediation and guarantee of the capital, or is it better to pay him a lump sum for his services?

There are issues related to Mudarabah (partnership):

1. It is not permissible for the capital of the Mudarabah to be unminted gold (ingots or jewelry). 2. If the agent mixes his money with the Mudarabah money, the amount of capital of each party must be known before mixing. 3. All investment expenses are borne by the capital, and it is not permissible to stipulate that they be borne by the agent or partially by him. 4. A friend's brokerage fee for connecting you to an investor is permissible, provided that the fee is known and not a percentage of an unknown profit. 5. A friend guaranteeing your capital with the investor in exchange for compensation is not permissible.