The Prophet ﷺ used to send Abdullah ibn Rawahah (to Khaybar), and he would assess the amount of dates when they began to ripen before they were eaten (by the Jews). He would then give choice to the Jews that they have them (on their possession) by that assessment or could assign to them (Muslims) by that assignment, so that the (amount of) zakat could be calculated before the fruit became eatable and distributed (among the people).  

كَانَ النَّبِيُّ ﷺ يَبْعَثُ عَبْدَ اللَّهِ بْنَ رَوَاحَةَ فَيَخْرُصُ النَّخْلَ حِينَ يَطِيبُ قَبْلَ أَنْ يُؤْكَلَ مِنْهُ ثُمَّ يُخَيِّرُ يَهُودَ يَأْخُذُونَهُ بِذَلِكَ الْخَرْصِ أَوْ يَدْفَعُونَهُ إِلَيْهِمْ بِذَلِكَ الْخَرْصِ لِكَىْ تُحْصَى الزَّكَاةُ قَبْلَ أَنْ تُؤْكَلَ الثِّمَارُ وَتُفَرَّقَ  

When Allah bestowed Khaybar on His Prophet ﷺ as fay' (as a result of conquest without fighting), the Messenger of Allah ﷺ allowed (them) to remain there as they were before, and apportioned it between him and them. He then sent Abdullah ibn Rawahah who assessed (the amount of dates) upon them.  

أَنَّهُ قَالَ أَفَاءَ اللَّهُ عَلَى رَسُولِهِ خَيْبَرَ فَأَقَرَّهُمْ رَسُولُ اللَّهِ ﷺ كَمَا كَانُوا وَجَعَلَهَا بَيْنَهُ وَبَيْنَهُمْ فَبَعَثَ عَبْدَ اللَّهِ بْنَ رَوَاحَةَ فَخَرَصَهَا عَلَيْهِمْ  

Ibn Rawahah assessed them (the amount of dates) at forty thousand wasqs, and when Ibn Rawahah gave them option, the Jews took the fruits in their possession and twenty thousand wasqs of dates were due from them.  

أَنَّ الْيَهُودَ لَمَّا خَيَّرَهُمُ ابْنُ رَوَاحَةَ أَخَذُوا الثَّمَرَ وَعَلَيْهِمْ عِشْرُونَ أَلْفَ وَسْقٍ