Is a person who had money stolen from him, which he was entrusted with to deliver to a charity, obligated to guarantee it, and is he sinful for his delay in delivering it?

The money held by a person with the permission of the Lawgiver (Shari'a) or the owner is considered a trust (amanah). A trustee (amin) is not liable for what perishes while in their possession unless there is an act of transgression (ta'addi) or negligence (tafrit) on their part. If a deposited item perishes without transgression or negligence from the depositee, then there is no liability upon them; because they took possession of the money with the owner's permission, and they are a doer of good (muhsin), and "there is no blame upon the doers of good." The difference between transgression and negligence is that transgression is doing what is not permissible, and negligence is abandoning what is obligatory. In your case, if the reason for the delay was a genuine excusable impediment, then there is no liability upon you. However, if the delay was due to laziness or carelessness on your part, then you are liable and must pay the amount to the association.