Is it permissible to subscribe to the Tasahil installment system, which involves a sum of money being deposited into a non-refundable electronic wallet that cannot be withdrawn as cash, but can be used to purchase devices whose value is then paid back to them in installments?

If "Tasaheel" grants you a loan to purchase appliances, with the condition that you repay it with an increase, then this is a forbidden usurious loan, whether you use the money or commit to purchasing the appliances with it. A usurious loan is unanimously prohibited, and its perpetrator is cursed. The permissible scenario is for "Tasaheel" to purchase the appliance for itself and take possession of it, then sell it to you in installments, provided that the contract does not include a late payment penalty, and that you do not sign the contract or pay an advance before "Tasaheel" has actually purchased the appliance.