In the late middle ages the banking system in Europe was developed enough to the point that the requirement to physically move purses full of gold florins was no longer a requirement.

Let's say for example that you were a silk merchant in Florence, and you needed to buy saffron from Spain to dye your cloth. You could give the money to a local florentine banker, who would record it as a credit in your account, send a letter to their bank representative in Spain, who would then record the transfer to the account of the Spanish merchant. You would be charged a fee for the transaction, depending on the number of days that it would take on average for the messanger to bring the letter recording the transfer (sent along with other business correspondence) to arrive at its destination. The Spanish merchant could then use the credit on his account to repeat the transaction to buy wool cloth from the Flanders from a Flemish merchant, again with no physical gold florins being moved, and a fee paid based on the number of travel days between Spain and the Flanders. Finally, the Flemish merchant could start a third fund transfer back to Florence, to buy some of your newly dyed yellow silk from you. In all these transfers, gold coins never moved, and in the end the Florentine bank and its branches made money by essentially holding the funds in their safe for a period of time on your behalf and sending messangers with instructions to register fund transfers into the ledgers.

Incidentally, if you see this whole process as essentially a transfer of gold coins from one person in Florence to another person also in Florence, coins that get returned a number of days later, minus a fee based on the number of days, you have essentially created what amounts to a loan with interest charged, which was something that was technically not allowed by the Church. Calling it a fee for transferring funds from one place to another and back to the original location and charging a "fee" was a loophole around the prohibition. Eventually the whole thing became just a matter of recording these loans in a ledger, and there was no longer an actual transfer of a paper document from a branch to another.

You could also do esentially the reverse transaction. Let's say that you had a ship full of wheat to send from Messina to Genoa. You would contact the local bank branch, use the ship and cargo as a guarantee and get funds recorded into your account, minus the usual time-based fees and most likely the payment of accident insurance that would cover the cargo in case of a shipwreck. Once at destination, the buyer would give the local branch the funds (either in cash or removed from his credit with the bank) and the transaction would be completed.

There are a lot of commercial and banking records extant from the central and late middle ages, starting around the year 1200 in Genoa and later in Florence and elsewhere in Europe. An example is a promissory note from 1099 Messina that is in the Internet Medievval Source Book. An excellent book on the topic of medieval commerce and banking is Goldthwaite - The economy of renaissance Florence.