How should one deal with a Sufi uncle, who insists upon his belief in saints and seeking help from other than Allah, given the existence of work, neighborhood, and kinship relations, and is it permissible to contribute to the salary of the mosque’s imam whom this uncle appoints, knowing that the imam's creed is sound, but he might put someone else forward to lead the prayer?

Your uncle's mere affiliation with a Sufi order does not mean he is a polytheist (mushrik). Drawing near to saints and seeking aid from the Prophet, peace be upon him, can be summarized as follows: some of it is polytheism (shirk), such as directing acts of worship to them; some of it is a means to polytheism, such as asking for their supplication; and some of it is a matter of scholarly disagreement, such as tawassul (seeking intercession). Do not rush to declare him a polytheist. The default assumption is that his affiliation with Islam is sound. If you are certain that he commits acts of polytheism, advise him and teach him gently and kindly. Provide him with pamphlets and booklets, and seek help from people of knowledge. There is no harm in giving money to the mosque imam or engaging in commercial transactions with him. However, if he insists on wrongdoing and there is a benefit in boycotting him, then boycotting him is permissible.