It is to be made on the spot. This is what I heard (from Allah's Messenger (may peace be upon him ).
Sahih Muslim Book 22, Hadith 112
← Previous Hadith I asked al-Bara' b. Azib about the exchange of (gold for silver or vice versa), whereupon he said: you better ask Zaid b. Arqam for he knows more than I. So I asked Zaid but he said: You better ask al-Bara' for he knows more than I. Then both of them said: Allah's Messenger (ﷺ) forbade the sale of silver for gold when payment is to be made in future.

Sahih Muslim Book 22, Hadith 111 Next Hadith → Allah's Messenger (ﷺ) prohibited us. The rest of the hadith is the same.

Sahih Muslim Book 22, Hadith 113