Imam Mawlud says that if a person feels rancour toward a particular person, he should show that person goodwill. By nature, people are naturally inclined to love those who do good to them. And if one shows a person good, feelings of rancour will fall to the wayside.
If a person has rancour toward another believer, God shall not forgive that person until he forgives his brother, for rancour is a serious affliction that festers in one's heart and blocks good things from coming to one.
"Purification of The Heart" - Hamza Yusuf, pp.122, 123