What is the ruling on using the electronic "Sadad" service for mobile phone purchases, which imposes a 3% deduction on each purchase, especially given the problem of cash liquidity in Libya?

If a person pays the price directly from their account without a loan from the telecommunications company, there is no harm in that. The fees that the company takes for opening an account and subscribing to the service, or for paying for purchases with it, whether a fixed amount or a percentage of the purchase price, are permissible because they are in exchange for services rendered and do not entail any religiously prohibited matter.