This question was asked on Quora (see the post here), and we couldn't resist diving in! Use of Tulips as currency is actually somewhat misleading for a few reasons. For one, there are a lot of historical inaccuracies related to Tulip Mania - often driven by the post-mania period when satirists and others sought to portray the craze in as negative a light as possible (for example, implications that merchants were frequently selling everything they owned in order to get a single Tulip bulb - this may have happened but it is not historically documented!).
What we do know (and what is relevant to the question) is the following: The height of Tulip Mania was during the winter months of 1636–1637. The winter also happens to be when Tulip bulbs are dormant underground (preparing to sprout in the Spring), and so no physical Tulips could be traded. As a result, all trading actually occurred with ‘contracts’, where instead of buying a bulb someone was buying the right to acquire that bulb after it bloomed in the Spring. It is also documented that these contracts often changed hands multiple times in a day (for example, someone buys it for one price in the morning, then sells it at another in the afternoon, and that person then sells it again in the evening).
It is easy to see how this might have created the mania we hear about today - everybody saw these pieces of paper, that were constantly going up in price, and that were very much ‘liquid’ as they traded several times a day. They wanted in!
And this is also where one could see these contracts (not the bulbs themselves) being used as a form of currency. If your friend owed you money, but only had one of these contracts, you might be happy to take it as you could easily sell it later that day.