Ustādh I did not understanding what you mentioned of Bay’ as-Salam and its difference to selling what you don’t have . Could please explain it very clearly like to a beginner. What is الموصوف في الذمة?

What the Prophet ﷺ forbade is selling a particular thing you do not own and cannot deliver, and salam is not that, it is a described obligation in your dhimmah, a known kind, measure, weight and quality to a known term, with the price fixed at the time of the contract. Al-mawsuf fi al-dhimmah means what is owed by description rather than a particular item pointed at, so dates of a stated grade and weight to be delivered in six months is salam; this is exactly like ordering something online and having it delivered. What is forbidden is that you take money for something you do not even own and then you go buy it and then you deliver it; Islam protects your wealth and prevents trouble, so what if the person takes your money and then can't buy it or delayed and does not know when he will be able to buy it etc... this will lead to a lot of problems and corruption so Islam closes the door altogether.