Michael Cohen outlined a crime. Did he prove that Trump committed one?

The most an American can contribute to a presidential (or any other federal) campaign is $3,300. The purpose of this limit is in part to prevent campaigns from costing enormous amounts of money from people to whom the candidate might feel beholden. Contributions and expenses are reported so the public can see who is funding campaigns and how the money is being spent. All of this is codified in federal law.