One way to understand how this works is to consider 2 parties in the following transaction. Let's say I offer as a package to: buy stock A at $200, sell stock A at $100. A 2nd party, the buyer, can see that there is $100 intrinsic/immediate value in this package. They are willing to buy it for $100.
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Now let's say I offer the same package, but I introduce a delay: you must wait 1 year to collect on this money. From the buyer's perspective, there is still the $100 value of the package, but they have to wait 1 year. There should be a cost to that waiting. So instead of offering $100 for the package, they offer $95. In other words, they are charging me $5 interest. That is a how a box spread works.
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Now let's say I offer the same package, but I introduce a delay: you must wait 1 year to collect on this money. From the buyer's perspective, there is still the $100 value of the package, but they have to wait 1 year. There should be a cost to that waiting. So instead of offering $100 for the package, they offer $95. In other words, they are charging me $5 interest.
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I can accept that offer, or look for a better deal. Many buyers will make offers, and generally the best offer will be close to the risk-free interest rate, which will usually be much less than borrowing money from your broker. That is a how a box spread works.
[Boxtrades](https://www.boxtrades.com/) is a good site for figuring out the trade to enter. You can even combine this with a forex carry trade to take the loan in another currency with lower interest rate, but you will need to be able to trade options in non-US markets (e.g. [SMI index](https://www.six-group.com/en/market-data/indices/switzerland/equity/smi.html) for CHF).