What is the ruling on giving money as an incentive to complete a certain task (filling out surveys or entering an email address), with the marketer receiving the difference between his profits and the paid incentive, knowing that the company's fee is known?

If the marketer agrees with a company to market its services, and he is paid a known fee, and the company's activity is permissible, then there is no harm in him offering incentives to those he brings in as customers or participants in surveys. This applies whether the marketer pays these incentives from his own money as a donation or if they are paid by the company, as long as this falls within permissible means of attracting customers without deceit or fraud.