Is it permissible to offer greetings or congratulations for Eid before the Eid prayer? And what is the evidence for or against its permissibility?

There is no evidence prohibiting Eid greetings before the prayer. Sheikh Ibn Uthaymeen mentioned that Eid greetings were exchanged by some Companions, and that they have become a customary practice among people to congratulate each other on the arrival of Eid and the completion of fasting.

When Sheikh Al-Fawzan was asked about prohibiting Eid greetings a day or two before Eid, he replied that he was unaware of such a statement, and that greetings are permissible on Eid day or thereafter. However, he was unaware of such greetings occurring among the predecessors before Eid day, indicating that it is not possible to congratulate on something that has not yet happened.

It appears from some narrations that the predecessors used to congratulate each other after the Eid prayer by saying: "May Allah accept [it] from us and from you." It was reported from Muhammad ibn Ziyad that he was with Abu Umamah Al-Bahili and other Companions, and when they returned from Eid, some of them would say to others: "May Allah accept [it] from us and from you." Imam Ahmad indicated the good chain of narration for Abu Umamah's hadith.

It is stated in Sunan Al-Bayhaqi that Khalid ibn Ma'dan met Wathilah ibn Al-Asqa' on an Eid day and said to him: "May Allah accept [it] from us and from you." Wathilah replied: "Yes, may Allah accept [it] from us and from you." Wathilah also mentioned that he met the Messenger of Allah, peace and blessings be upon him, on an Eid day and said that to him, and the Prophet responded with the same words.

No prohibition has been reported concerning greetings before the prayer or restricting them to a specific time.