Is it permissible to work in a mobile phone shop that was rented from its original owner for a certain amount, and then rented from the first tenant for a higher amount after the first tenant made renovations to the shop?

It is permissible for the tenant to rent out the leased property to another person, even for more than what he himself rented it for. This is the view of the Maliki, Hanbali, and Shafi'i schools, as long as the sub-tenant is similar to him in condition, trustworthiness, and craftsmanship.