Is it permissible to demand a guarantee for the principal of a stalled project, based on a prior condition for a guarantee case, despite knowing that there is no guarantee for the principal in a lawful Murabaha?

If the guarantee of the working brother (mudarib) for the capital of the investor (sahib al-mal) is absolute, then this condition is invalid. This is because it takes the Mudarabah contract out of its Sharia-compliant form and transforms it into a loan, which makes the distribution of profit usury (riba). This does not fall under the rule "Muslims are bound by their conditions," because that applies only to conditions that are permissible according to Sharia. As for the rule "the guarantor is liable" (al-za'im gharim), it applies to lawful rights. So, if the mudarib is obliged to pay something due to his negligence or betrayal, then his guarantor is also obliged to do so.