Is the customer's money remaining suspended in the payment system for 21 days considered a قبض (possession)? And does setting the shipping period between 7 and 15 days comply with the condition of a known term in a Salam contract, taking into account the possibility of delay?

The first matter has been answered previously in fatwas: 162951, 104072.

As for the second matter, regarding the imprecision of the term (deadline), it is problematic. However, some scholars permit a slight imprecision, especially if it does not lead to a dispute.

Ibn al-Arabi narrated from Malik the permissibility of selling goods until the time of picking (dates) or harvesting (crops), considering this a known term. In contrast, other jurists considered it unknown. Nevertheless, Ibn Qudamah mentioned that Ahmad ibn Hanbal, Malik, and Abu Thawr permitted it.

Based on this, the scenario mentioned by the questioner, concerning the ambiguity of the term, is potentially valid, especially according to the Malikis and in one narration from the Hanbalis.

It is more prudent to specify a precise term that ensures the delivery of the goods. Jurists have stipulated that the buyer is obligated to take possession of the forward-sold good before its due time if the seller delivers it and there is no harm to the buyer in doing so. This indicates the permissibility of delivering before the agreed-upon time.