Is it permissible for a Mudarabah or Musharakah merchant to dispose of the collected checks' funds during the transaction period for his personal benefit or to finance other private transactions of his, or to inform the financier of a longer period for the transaction to end so he can use the money during this period, or to stipulate to him the delay of returning the money after the transaction ends for his private use?

If the financier pays with post-dated checks, the Mudarabah is invalid because the capital is stipulated to be cash, not a debt. It is permissible to restrict the Mudarabah to a specific period or a particular commodity. It is not permissible for the merchant to dispose of the financier's capital for his own benefit during or after the Mudarabah, as this is considered a betrayal and an unlawful appropriation of another's money. Mudarabah is based on agency and trust, and the merchant, in such a case, is an usurper and an aggressor. Mudarabah has conditions that must be adhered to, including that the merchant's capital must be known and distinguishable if it is mixed with the financier's capital, that the share of the two partners in the profit must be a common share, and that the financier's capital must not be guaranteed by the merchant except in cases of negligence or transgression.