"Extra" and "additional" mean the same thing. However, in this context, it's more common/less formal to use "extra." Also, "assist" is more formal than "help." Now, I have a correction for you. When you make a negative statement, use "any" before the noun. For example, "I don't have any additional money." "I don't have any extra money."

Extra is more of the same while additional is more of other things. If you have extra tools, that would mean that you have more of the same tools. If you have additional tools, then you have more than just one kind of tools. So for those sentances you would say "I don't have any extra money" (I hope this isn't too confusing, the two words are very similar)