Is permissible to buy/sell with non-Muslim?
                                Yes buying/selling for a Muslim with non-Muslim is permissible as regards what is lawful.
                             
                        
                            
                                
                                What is the best remedy for a person who experiences Waswasah (whispers of the devil) during Salah (prayer)?
                                Such a person should ignore the devil`s whispers, contemplate the Quranic chapters and verses that he/she recites, look directly at the place of Sujood (prostration), make supplication to Allah, and take refuge in Him from the cursed devil.
                             
                        
                            
                                
                                Is the one who gives up on marriage because he can`t afford it considered sinful?
                                Marriage is desirable, but one who can`t afford it isn`t considered sinful. However, he should supplicate to Allah, and seek His forgiveness.
                             
                        
                            
                                
                                I am a doctor, and sometimes I refer patients to the hospital for surgical interventions, and they give me a commission from the fees of the surgeries, even though I do not perform these surgeries. What is the ruling?
                                All perfect praise be to Allah, The Lord of The Worlds, and may His Peace and Blessings be upon our Prophet Muhammad and upon all of his family and companions.
If this commission is charged to the patient as part of the operation costs, then it is not permissible. Additionally, a doctor should only recommend surgery if there is a genuine medical need, and the hospital should only perform an operation when necessary. And Allah The Almighty Knows Best.