If the incentive costs are borne by your website and not the sponsor's, are offered in relation to the sponsor site's join, are not a rebate of the referred's direct costs in joining the sponsor's site, and are not made with the purpose of defrauding the sponsor somehow -- you probably could see no harm in it.

However, the seller (sponsor) becomes liable for his agent's (affiliate's) offer should the affiliate website offering the free credits to his website for the signup not be able to deliver as promised.

We wouldn't allow it for the aforestated reason.