(In order to understand the dinim of Ribis and if this particular case would be problematic I will give a localized example. However please don't close the question or say it's too "local" because the whole point in these questions is to understand the issur of Ribis better and more clearly and how it can be applied to other cases as well.)
A poor person went collecting Tzedeka on Purim. Towards the end of the day he came to a rich person who said that he had already run out of money and had no more to give that day. The poor person proposed the following. He said he'd loan the rich person 100 shekels now in order that he should be able to give it right back to him as tzedeka, and, after Purim, the poor person would come back and the rich person would pay back the poor person the 100 shekels that had been given as a loan. As I understand, if the rich person wouldn't be willing to give over the 100 shekels now as charity that the poor person had just lent him, then there would be no such arrangement to begin with.
Is this a problem of ribis?
Perhaps this similar to the case saying that you'll make a loan to someone only on condition that they give tzedeka to certain organization (which is a problem as I've heard -- no source at the moment). Or perhaps the 100 shekels in this case being given on Purim could be looked at as a matana (gift) since it's being given as Tzedeka.
(If anyone feels the question needs editing or addtional sources, please feel free to add or edit it yourself. I will bli neder try and go back and post more however I'm putting up the question now in order to hopefully get answers as well as improve upon it.)