Is it obligatory to uphold a covenant of friendship over which Allah has been made a witness, or is it permissible to break it, and what is its expiation, especially if suspicious behaviors emerge from the other party?

If you utter a vow (عهد) attributing it to Allah, such as saying, "It is a vow to Allah that I will do such and such," then breaking it necessitates expiation for an oath (كفارة يمين). However, if the vow is not accompanied by an oath, then breaking it is a moral wrong and does not necessitate expiation. Fulfilling a vow is obligatory unless it involves an act of disobedience (معصية), in which case it must be revoked.