Chametz must be set aside in a special place which is rented to the non Jew who has purchased it, so that the chametz becomes the property of the non Jew until after Passover.  
The bill of sale for the chametz states that he is selling his chametz to the non Jew for a specific price. The non Jewish purchaser then gives him a down payment either money or something with intrinsic value, to acquire ownership of the chametz.  

A stipulation is added to the bill of sale, stating that if the purchaser does not pay the balance due by the end of Passover, the chametz will revert to the original owner at that time that is, at the end of Passover. The non Jew's failure to pay will not be seen as having retroactively invalidated the sale.  

Although the seller is fully aware of the fact that the non Jewish purchaser will not meet the terms of the agreement by paying the full amount, and that the chametz will thus revert to his possession after Passover, nevertheless, when he sells his chametz before Passover, he must consider it sold and therefore no longer his.  
Transfer of ownership, according to Halachah, requires intent to sell; that is, the seller must intend to relinquish his rights to his property in order for title to be transferred to someone else and an act of acquisition must be made. Hence, when the seller consciously transfers ownership of the chametz, and when payment is made, the chametz can remain in the home of the Jewish seller since it is no longer his.  

If the price of the sold chametz rises during Passover, and the non Jewish purchaser decides to pay the balance due at the end of Passover so as to profit from the price rise, the Jewish seller has no recourse but to complete the sale by accepting the amount specified in the bill of sale.  

Today, it is customary for people to sell their chametz through the offices of the Rabbinical court. An individual who wishes to sell chametz appoints the Rabbinical court to act as his agent and sell his chametz, as well as the place where it is stored, to a non Jew.  
Only chametz that is visible need be sold to a non Jew. As for chametz like that which is absorbed in one's dishes, for example, it is sufficient to wash the dishes thoroughly and put them away in a place which one will not enter on Passover.  

Some authorities, however, maintain that this is not sufficient and include dishes in the sale to the non Jew. When these dishes revert to Jewish ownership after Passover, they do not require ritual immersion, which is the case with dishes purchased from a non Jew], for only the chametz absorbed within the dishes was sold and not the dishes themselves.  
Chametz may be sold to a non Jew for as long as one is permitted to derive benefit from it, i.e. until the fifth hour of the fourteenth of Nissan. After this time, one cannot sell chametz since it is no longer his, and he has no alternative but to destroy it, no matter how great the loss he incurs.  
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