Is it permissible for a widow to refrain from selling an inherited apartment until her minor daughters reach maturity, and is she entitled to negotiate with the heirs to receive an "empty property" payment (khalu 'an al-ijar) in addition to her right to the inheritance?

You do not have the right to prevent the heirs from selling the apartment as long as it is not owned by you or your husband, unless your husband had paid its rent. In this case, you have the right to reside in it for the duration of the 'iddah (four months and ten days, or until delivery if you are pregnant), or for the remainder of the lease term. The heirs are not permitted to evict you from it, because the woman whose husband has passed away, if he had consummated the marriage with her, has the right to reside in the husband's home, or in a home for which he paid the rent. The right of dwelling is due to the woman whose husband has passed away if he had consummated the marriage with her, and the dwelling belongs to him or he paid its rent in cash. If the rent is paid monthly, then the condition of paying the rent in advance applies. However, if it is for a fixed term (a specific period other than a month), then the wife is entitled to dwell in it whether the husband paid the rent in advance or not. You are not permitted to leave the apartment except for a need or necessity. If your continued stay in the apartment results in harm, then there is no objection to leaving and taking compensation (khalw) from the heirs or others. You can offer to buy the apartment from the heirs in exchange for your share and your daughters' shares from your husband's inheritance, but you do not have the right to refuse the sale or to leave the apartment without a legitimate Shar'i justification.