Is commission-based income (e.g. real estate brokerage, MLM) always halal?

No, not automatically — it depends heavily on the structure. Standard commission-based income, like a real estate broker's fee for genuinely connecting a buyer and seller, or a salesperson's commission on real sales they facilitated, is generally halal, since it's payment for a real service (this is the classical concept of ju'alah or a wikalah bi ujrah — an agency contract with a fee), provided there's no deception, hidden dual-dealing, or misrepresentation involved.

MLM (multi-level marketing) schemes are a different matter and are where most of the caution belongs — many contemporary fatwa bodies and scholars (including analyses from the Fiqh Council of North America and various national Islamic councils) have specifically ruled that MLM structures where income depends primarily on recruiting new participants rather than genuine retail sale of a product resemble gambling (maysir) and involve real deception (gharar/tadlis), since most participants mathematically cannot profit in a pyramid-shaped structure regardless of effort. If a specific MLM's income is genuinely and primarily earned through real product sales at fair prices, with recruitment commissions being incidental rather than the actual engine of income, some scholars would view it more leniently — but readers should scrutinize the actual compensation plan of any specific MLM they're considering rather than assume "it's just sales" applies.

Source: general fiqh on ju'alah/agency commission as halal; MLM-specific concerns documented in Fiqh Council of North America and Majlis Ugama Islam Singapura (MUIS) advisories on multi-level marketing.