The firewall, in one sentence
Profit is a share of what the venture actually earns; capital is genuinely at risk; and there is no guaranteed return. Hold those three together and you have a risk-sharing partnership, not a loan.
What's structured in
Profit shared by a pre-agreed ratio on realized profit. Capital at risk, with loss borne by the capital provider. The manager liable for misconduct, negligence, or breach. Terms that Shariah fixes kept out of negotiation.
What collapses it into riba
A fixed or guaranteed profit amount to either party. Profit quoted as a percentage of the capital (a yield). Guaranteed or 'protected' principal. Any framing as a loan, lending, or interest.
Each of these reintroduces a predetermined return on money regardless of outcome — which is exactly what riba is.
Why the language matters
Even with the right structure, careless wording — 'expected 8% yield,' 'capital protected' — can imply a guaranteed return. Describe profit as a share of realized profit, and be honest that capital is at risk. The honest description is also the compliant one.