Is it obligatory for one of the heirs to pay off the debt of their deceased father, which the heirs are unable to pay, or should the money be used for marriage, knowing that the merchant is demanding his debt?

We ask Allah to have mercy on your father and admit him into His spacious Gardens. Regarding your question about whether to prioritize the deceased's debt or marriage, the answer is in Fatwa No. 44261. As for whether the deceased's obligation is cleared merely by the heirs or others taking on his debt, or if it is only cleared by actual payment, there is a difference of opinion on this issue; please refer to Fatwa No. 131062.