Quite simply, 50g will be added to your debt. It's triggered by the fact of being negative in gold when you leave the fountain.
Thus every time you would make a purchase that would put you in the negative, 50g is slapped onto that debt. This extends your ability to buy expensive items, with less actual gold. However, this comes at the cost of losing money for it.
The first loan being free means that when you go in the negative for the first time, you won't get a 50g fee slapped on. Thus you can go back and get a
Needlessly Large Rod with 1100g and your debt would only be -150g. However, if you tried again with 1100g afterwards, your debt would be -200g. Which means you paid 1300g for that second rod.
So you essentially need to farm up more to make up for that deficit. If having the **FIRST ** item quicker would mean you could be making more money quicker, then that is the optimal purchase for Future's Market. The main benefit is that allows you to obtain items much quicker at the price of 50g for every item you would go negative purchasing.