What is incumbent upon a person, and what is their duty to do, if they are entrusted with money to deliver to the poor and needy, but they delay in sending it, spend some of it, and then return it, all without the knowledge of the owner of the money?

Your disposal of funds allocated for a specific entity, or withholding them from their rightful owner without a valid excuse, is considered a breach of trust (amanah), which is forbidden by religious law and is a sign of hypocrisy. It is obligatory to repent to Allah by feeling remorse and resolving not to return to such an act, and to immediately pay all the money to its rightful owner. Repentance is not valid without doing so, for "the hand is responsible for what it has taken until it returns it."