In the context of the principal-agent relationship, the best strategy to pursue would probably be the one that involves identifying partner organizations in the region, then monitoring and enforcing the responsible humanitarian assistance. This strategy gives the principal lots of power. Instead of relying on one institution, like the United Nations, or one country, the Republic of Congo, the first strategy affords the principal the greatest amount of flexibility and command.
The principal can choose the organizations that will reflect its beliefs about humanitarian assistance. As these agencies are disparate and separate, the principal has a better chance of ensuring that they will act on the principal’s behalf and don’t deviate from their specific agendas.
Generally, the United Nations (UN) would not be considered a good agent for any one country. At various times, this intergovernmental body has upset myriad countries (principals) for various reasons. In 2018, the UN infuriated the United States when the UN Security Council drafted a resolution that criticized Israeli violence against Palestinian citizens. In the early 2000s, the UN declared that the Iraq War was illegal, much to the chagrin of the United States and the United Kingdom. These examples demonstrate that the UN is inclined to act independently, not merely as an agent for a certain country.
As for giving a country the money directly, that doesn’t always tend to work out so well. When a country gives another country money, that doesn’t guarantee that they will act on their behalf.