Coun Shaffaq Mohammed said he wanted to know more about more than £1.5m of taxpayers’ money being handed to a pension fund and a private company.
Some £900,000 is being given to the Medical Research Council Pension Fund to help turn the former Topman store on Fargate into offices.
And £750,000 is set to be handed to London Associated Properties to convert disused space in Orchard Square into flats and improve the ‘public realm’.
Both organisations say the projects would be ‘unviable’ without public cash.
A further £3.4m is available for similar schemes.
Coun Mohammed wondered if the grants were ‘over generous’ and whether ‘we could have spread the jam a bit more’.
He also highlighted the £1.5m redevelopment of the former Next building on Fargate by Wakefield-based Woodhead Investments which was viable despite not receiving public cash.
He added: “They are eye-watering sums. Are we subsiding pension funds for their members and a private company to benefit investors?
“How much are they putting in? It’s always a balance, but money doesn’t grow on trees. In the interests of openness and transparency I want to know what they are doing.”
A council report states the former Topman is being converted into a four-storey office.
In Orchard Square some 13 flats are set to be created, as well as a covered events space.
A London Associated Properties spokeswoman said the project would not go ahead without the grant because the company would lose money because the value of the property, when completed, would be less than that spent on it.
The firm plans to rent the flats out, she added.
Orchard Square was included as part of the ‘bid area’ which includes Fargate and High Street, she added.
The Medical Research Council and Sheffield City Council were contacted for comment.
A council report on the MRC Fargate building states: ‘The Future High Streets Fund will be used to fund the identified viability gap of up to £900,000. This will be achieved through a grant funding agreement with the building owners’.