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

نَهَى رَسُولُ اللَّهِ ﷺ عَنِ الْفِضَّةِ بِالْفِضَّةِ وَالذَّهَبِ بِالذَّهَبِ إِلاَّ سَوَاءً بِسَوَاءٍ وَأَمَرَنَا أَنْ نَشْتَرِيَ الْفِضَّةَ بِالذَّهَبِ كَيْفَ شِئْنَا وَنَشْتَرِيَ الذَّهَبَ بِالْفِضَّةِ كَيْفَ شِئْنَا قَالَ فَسَأَلَهُ رَجُلٌ فَقَالَ يَدًا بِيَدٍ فَقَالَ هَكَذَا سَمِعْتُ  

[Machine] Abu Bakr prohibited us, the Messenger of Allah, from buying silver with silver and gold with gold, except like for like. And he ordered us to buy silver with gold and gold with silver in whatever way we wished. Thabit ibn Ubaid asked him, "Hand in hand?" He said, "That's how I heard it."  

أَبُو بَكَرَةَ نَهَانَا رَسُولُ اللهِ ﷺ أَنْ نَبْتَاعَ الْفِضَّةَ بِالْفِضَّةِ وَالذَّهَبَ بِالذَّهَبِ إِلَّا سَوَاءً بِسَوَاءٍ وَأَمَرَنَا أَنْ نَبْتَاعَ الْفِضَّةَ فِي الذَّهَبِ وَالذَّهَبَ فِي الْفِضَّةِ كَيْفَ شِئْنَا فَقَالَ لَهُ ثَابِتُ بْنُ عُبَيْدٍ يَدًا بِيَدٍ؟ قَالَ هَكَذَا سَمِعْتُ  

[Machine] Abu Bakrah narrated that the Messenger of Allah ﷺ prohibited us from trading silver with silver and gold with gold, except equally. And he commanded us to trade silver with gold and gold with silver as we wish. So Thabit ibn Abdullah asked him, "Hand in hand?" He replied, "Yes, that is how I heard it."  

أَبُو بَكَرَةَ نَهَانَا رَسُولُ اللهِ ﷺ أَنْ نَبْتَاعَ الْفِضَّةَ بِالْفِضَّةِ وَالذَّهَبَ بِالذَّهَبِ إِلَّا سَوَاءً بِسَوَاءٍ وَأَمَرَنَا أَنْ نَبْتَاعَ الْفِضَّةَ فِي الذَّهَبِ وَالذَّهَبَ فِي الْفِضَّةِ كَيْفَ شِئْنَا فَقَالَ لَهُ ثَابِتُ بْنُ عَبْدِ اللهِ يَدًا بِيَدٍ؟ فَقَالَ هَكَذَا سَمِعْتُ