Is my refraining from speaking to a person who took money from me and promised me a job opportunity considered forbidden abandonment, knowing that I feel intense hatred towards him? Is it permissible for me to supplicate against him? Is my forgiveness of him valid if I entrust my affair to Allah but will not speak to him again?

It is not permissible to abandon a Muslim for more than three [days], unless there is a fear that speaking to him will corrupt one's religion or cause harm to oneself or one's worldly affairs. If you greet him upon meeting him, your aversion to conversing with him is not considered abandonment. It is permissible to supplicate against an oppressor, but patience and forgiveness are better. Forgiving him and entrusting the matter to Allah is a sign of strength, not weakness.