Is working online to market non-forbidden products, which requires paying a refundable amount after six months (with a 30% deduction in case of early withdrawal), and offers rewards for adding and supervising new members, permissible in Islamic law?

It is not permissible to take money from a worker as a guarantee of their seriousness, whether for use or not, because it falls under the category of a loan and exchange, which is forbidden. Every loan that draws a benefit is prohibited usury (riba). If a deduction is made from it upon the worker's withdrawal, it constitutes consuming wealth unjustly, even if both parties agree to it, because there is no justification for taking a pledge or security from the worker. Giving a commission or gift with the possibility of a deduction from the amount paid makes the matter gambling. The correct solution is to cancel the upfront payment and then give gifts or commissions for bringing in others or for marketing.