CB Richard Ellis executive director Stuart Robinson said the proposal, outlined by London mayor Ken Livingstone last week, could leave less cash for project elements such as affordable housing.
The mayor said agreements could help provide subsidised and below-market rents for shops that could not otherwise afford them.
But Robinson suggested that it would be better to ensure that schemes contain smaller shops with lower rents to draw independents.
Robinson also questioned how "local traders" could be defined, monitored and enforced. Livingstone proposed linking the term with section 106 agreements in draft revisions to the London Plan.