fault

Q: I purchased a brand-new stove last year from a Muslim brother who owns an appliance store. Two weeks ago, the thermostat blew. When the electrician opened the stove and was changing the thermostat, he noticed that the brand-new parts of the stove had been replaced with old, slightly damaged parts before it was sold to me. When I complained to the appliance store, they acknowledged the fault but were unhappy to give me a full refund because of the blown thermostat. What options do I have in Shari’ah?

Q: When I was around 13 to 14, I attempted to modify someone's console but it became faulty. They said don't worry about it we'll get it replaced. Am I sinful, although physically I was in puberty but not mentally.