Praise be to Allaah.
Firstly, it is not permissible for you to take zakaah on their behalf, because they have enough money of their own. Zakaah is for the poor and needy, not for orphans. Whatever you have taken at the time when they had enough of their own has to be given back to the original donors, if you know who they are, otherwise you must give it away in charity with the intention of it being zakaah on their behalf, because you took it with the intention of it being zakaah from them.
As for the money that you have accumulated from the retirement fund, use it as you see fit, because Allaah says (interpretation of the meaning): “And come not near to the orphan’s property, except to improve it…” [al-An’aam 6:152].
As for the debt to the real estate development company, you know that it is to be repaid in installments, not all at once, so pay it back in installments. The deceased has nothing to do with it, except with regard to the installments that were due before his death, if he did not pay them. As for the installments that were not due until after his death, they are nothing to do with him because they have to do with this property, the ownership of which has passed to his heirs. So they are responsible for these repayments, and they should not be paid from the zakaah money, because they have enough money of their own with which to pay them.