Its interesting because advance sales are a positional bet similar to a short sell. Are you allowed to sell the Salam sale contract to a third party? Apologies for all the questions this is fascinating.
The same applies to more elaborate schemes that involve multiple parties or transactions. For example, it is possible to engage in a futures contract with the intention of gambling (buy oil or some commodity, and never take delivery of it and sell it immediately when the contract is due). This is gambling, your intention was never to actually possess the item, but only to immediately sell and either make a loss or profit.
Furthermore, goods for which the quality and quantity cannot be specified cannot be sold by Salam contracts, as per the Hadith in my previous post (e.g. gems or precious stones since each is considered different), and I've seen at least one source that also exempts stocks from being tradable in Salam contracts due to high uncertainty (called Gharar sales in Islam, also prohibited).
As far as selling Salam contracts before they are fulfilled, I do not know. What sorts of issues or benefits can result from selling such a contract before it is fulfilled?
Edit: I think it is prohibited as well, as now the buyer is selling something that he does not yet possess. So you have the same item being sold multiple times before taking delivery. This narration should be the basis of prohibition:
> The Prophet (peace and blessings of Allah be upon him) forbade selling goods in the place where they were bought until the merchants had moved them to their own location.
> Apologies for all the questions this is fascinating
No problem! I'll do my best to answer what I know.
[0] https://practicalislamicfinance.com/2020/02/17/short-selling...
Islam prohibits something called Riba. Interest and usury fall under it, but Riba encompasses more than just loans. The example about purchasing gold on credit falls under riba for instance.