Is the expiation of "tasdeeq" (atonement by giving charity) sufficient for not speaking to a young man for one instance, or for each instance? And is it possible to speak to him as if the oath had never been taken?

You must immediately sever your relationship with this young man and refrain from speaking to him or corresponding with him, as this is a forbidden door to temptation. You must hasten to repent to Allah Almighty by abandoning this sin, regretting it, and resolving never to return to it. You must also expiate the oath you broke by feeding ten needy persons or clothing them. Fasting is not sufficient unless you are unable to feed them.