Is a signed business fax, proof of a business agreement s

It could be. The issue is whether there was an offer, and acceptance and consideration. Faxes have been used for many, many years and have become an acceptable way of conveying information, such as intent to enter into an agreement. An agreement can formed verbally; one can certainly be formed by fax, which is much more certain than an oral agreement. There may be other facts, issues and nuances be considered to give you a definitive answer to you questions. You should take the fax documents to an attorney and get a definitive opinion.