Suppose that Reuven is strapped for cash. He goes to the local Gemach and gets a hefty $10,000 dollar loan for his daughter's wedding. 3 months later, Reuven manages to acquire the money and repays his debt.
Reuven then feels a tremendous gratitude towards this Gemach and wants to show his appreciation by donating some money. Is Reuven allowed to donate money to the Gemach, or is this a case of Ribbis?
[I am purposely leaving the time-span between Reuven's repayment and his feeling of benevolence out of the case. I would expect an answer cover all possible time-spans, and how they relate to the Halacha]