Yes, if you made an offer to the artist and the artist accepted, that may well create an enforceable oral (often called verbal) agreement even in advance of a written contract being signed. The terms can be adjusted after acceptance of the original offer, but only if both parties agree or consent to the change (contracts may only be changed by mutual consent).Note that the exact circumstances--what specifically was said by each side--are critical in determining whehter, in this particular case, a contract was formed. For example, if you said, "I will pay you $10,000 to perform on November 3, 2015) and the artist said "agreed," that would form a contract. But if you just said, "would it possible to hire you to perform, maybe on the 3rd of November" and the artist said "yes," that is most likely NOT a contract--there was not sufficient definiteness as to what was being agreed to, since all the artist did was provide the information that they are for hire and may be available on the 3rd, and all you did was inquire into whether hiring them for that day was feasible. Neither of you, in this second example, committed to the hiring for that date.