Legality? It would be legal as there are no limits in the DR on the amount of money private citizens, regardless of nationality, or even companies, can give to the political candidate of their choice. Whether donation of very large sums of money to a Dominican campaign can decide who will be the next US ambassador in the DR is an unknown. Can a recently-elected President petition who will be the next US ambassador? Doesn't sound very likely. In the past, as far back as I can remember, US ambassadors have been persons unheard of in the DR prior to their appointment by the US government.
Now if that person is well connected to the next President of the United States, then that person could exercise influence in the US and be appointed ambassador. I understand that both Charlie Manatt (Clinton) and Hans Hertell (Bush) were political appointees. I do not know of any case in recent history when the President of the Dominican Republic opposed a nomination for ambassador of the US. It seems even more far-fetched that a recently-elected President of the Dominican Republic could endorse someone to become the next US ambassador in the US. Of course, in politics....