The life-tenancy which the Messenger of Allah ﷺ allowed was only that one should say: It is for you and your descendants. When he says: It is yours as long as you live, it returns to its owner.
إِنَّمَا الْعُمْرَى الَّتِي أَجَازَهَا رَسُولُ اللَّهِ ﷺ أَنْ يَقُولَ هِيَ لَكَ وَلِعَقِبِكَ فَأَمَّا إِذَا قَالَ هِيَ لَكَ مَا عِشْتَ فَإِنَّهَا تَرْجِعُ إِلَى صَاحِبِهَا