What is the Sharia ruling on agreeing upon fixed monthly profits (400 EGP) for an initial partnership (10,000 EGP) in a grocery store, knowing that the actual profits might be higher? Does this fall under the suspicion of usury (riba)? And how is the zakat due from the partner calculated in this case?

What is understood from the question is paying a sum of money to a person to buy goods and sell them for a share of the profit, which is a Mudarabah (profit-sharing partnership). For it to be valid, it is stipulated that no specific profit should be fixed, nor should the capital be guaranteed. If the capital is guaranteed or a known monthly profit (400) is fixed, then the Mudarabah is invalid. In this case, all the profit belongs to the owner of the capital, and the worker receives a wage equivalent to his work or a similar profit. This invalid contract must be dissolved, and all the profit belongs to the owner of the capital, while the worker receives a fair wage (اجرة المثل) or a similar profit. It is permissible for both of you to enter into a new Mudarabah contract in accordance with Sharia regulations.