Authorities like the Or Zaru'a have been so stringent as to say that even mental pledges are binding when it comes to gifts to the poor. Many less extreme opinions hold that verbal pledges of the same are binding. If one pledged a specific pile of money (specific bills or coins) to be given to an individual in need, would he be allowed to exchange that pile for a different one of like value?
I assume this could depend not only on the laws of gifts to the poor but also the laws of binding statements/oaths in general if one made a specific statement such as "I pledge to give this $50 to Mr. X."