Clearly taking any form of remuneration from a provider, that in anyway relates to the number of contractors using the service, could completely undermine any due diligence process.
If the agency was also insisting that contractors only use their PSL providers this could further undermine any due diligence process.
See also Q&A Can we restrict Contractors to use PSL companies only?
Under no circumstance should agencies allow remuneration or incentives from providers directly to their consultants, as this definitely leaves agencies exposed. We would suggest that any provider who still offers these direct incentives is encouraging non compliance with an agencies operating procedures.