It is to be made on the spot. This is what I heard (from Messenger of Allah (may peace be upon him ). (Using translation from Muslim 1590a)   

نَهَى رَسُولُ اللهِ ﷺ عَنْ بَيْعِ الْفِضَّةِ بِالْفِضَّةِ وَالذَّهَبِ بِالذَّهَبِ إِلَّا سَوَاءً بِسَوَاءٍ وَأَمَرَنَا أَنْ نَشْتَرِيَ الْفِضَّةَ بِالذَّهَبِ وَنَشْتَرِيَ الذَّهَبَ بِالْفِضَّةِ كَيْفَ شِئْنَا قَالَ فَسَأَلَهُ رَجُلٌ فَقَالَ يَدًا بِيَدٍ؟ فَقَالَ هَكَذَا سَمِعْتُ  

Dates are to be paid for by dates, wheat by wheat, barley by barley, salt by salt, like for like, payment being made on the spot. He who made an addition or demanded an addition, in fact, dealt in usury except in case where their classes differ. (Using translation from Muslim 1588a)   

قَالَ رَسُولُ اللهِ ﷺ التَّمْرُ بِالتَّمْرِ وَالْحِنْطَةُ بِالْحِنْطَةِ وَالشَّعِيرُ بِالشَّعِيرِ وَالْمِلْحُ بِالْمِلْحِ مِثْلًا بِمِثْلٍ يَدًا بِيَدٍ فَمَنْ زَادَ أَوِ اسْتَزَادَ فَقَدْ أَرْبَى إِلَّا مَا اخْتَلَفَتْ أَلْوَانُهُ  

[Machine] I heard the Messenger of Allah ﷺ saying, "Exchange gold for gold, silver for silver, wheat for wheat, barley for barley, dates for dates, and salt for salt, like for like, equal for equal, hand to hand. If the types differ, then sell as you wish, as long as it is hand to hand. Gold may be exchanged for silver however you please, wheat may be exchanged for barley however you please, and salt may be exchanged for dates however you please."  

سَمِعْتُ رَسُولَ اللهِ ﷺ يَقُولُ بِيعُوا الذَّهَبَ بِالذَّهَبِ وَالْفِضَّةَ بِالْفِضَّةِ وَالْبُرَّ بِالْبُرِّ وَالشَّعِيرَ بِالشَّعِيرِ وَالتَّمْرَ بِالتَّمْرِ وَالْمِلْحَ بِالْمِلْحِ سَوَاءً بِسَوَاءٍ مِثْلًا بِمِثْلٍ فَمَنْ زَادَ وَاسْتَزَادَ فَقَدْ أَرْبَى فَإِذَا اخْتَلَفَ هَذِهِ الْأَصْنَافُ فَبِيعُوهَا يَدًا بِيَدٍ كَيْفَ شِئْتُمْ لَا بَأْسَ بِهِ الذَّهَبُ بِالْفِضَّةِ يَدًا بِيَدٍ كَيْفَ شِئْتُمْ وَالْبُرُّ بِالشَّعِيرِ يَدًا بِيَدٍ كَيْفَ شِئْتُمْ وَالْمِلْحُ بِالتَّمْرِ يَدًا بِيَدٍ كَيْفَ شِئْتُمْ  

[Machine] Buy some barley with it and he went to take a Sa' (a measure of weight) and an additional amount of some Sa', so when Ma'mar came to know about this, he said to him, "Why did you do that?" "Go and return it, and only take a like for a like, for I used to hear the Messenger of Allah ﷺ say: 'Food is by a like for a like,' and our food at that time was barley. It was said to him: 'But there is nothing like it.' He said: 'I fear that it may be competed with.'"  

بِعْ ثُمَّ اشْتَرِ بِهِ شَعِيرًا فَذَهَبَ يَأْخُذُ صَاعًا وَزِيَادَةَ بَعْضِ صَاعٍ فَلَمَّا جَاءَ مَعْمَرٌ أَخْبَرَهُ بِذَلِكَ فَقَالَ لَهُ مَعْمَرٌ لِمَ فَعَلْتَ ذَلِكَ؟ انْطَلِقْ فَرُدَّهُ وَلَا تَأْخُذَنَّ إِلَّا مِثْلًا بِمِثْلٍ فَإِنِّي كُنْتُ أَسْمَعُ مِنْ رَسُولِ اللهِ ﷺ يَقُولُ الطَّعَامُ بِالطَّعَامِ مِثْلًا بِمِثْلٍ وَكَانَ طَعَامُنَا يَوْمَئِذٍ شَعِيرًا قِيلَ فَإِنَّهُ لَيْسَ مِثْلَهُ قَالَ فَإِنِّي أَخَافُ أَنْ يُضَارِعَ