The Sheriff's Department might assist you in retrieving the calf if you can prove ownership and the calf is still on his property. That is called a civil standbye, some departments do them and some don't. We did not do them with my department in Missouri as we did not get wrapped up in civil issues. But in Wyoming my department does do them. As far as I was speaking, there is nothing criminal with this situation. I'm sure there is not a written contract, so proving the calf is yours could be difficult if the calf is not branded. The Brand Inspector is a Law Enforcement official in Wyoming for livestock laws, so if that calf is branded and he does not have paperwork from you transferring that calf to him, getting it back would be easy or if he admits the calf is yours it would be easy. But on the other hand if he denies the calf is yours, then you could be in for a long drawn out issue.
Legal advice is good as I'm not an attorney and this is a civil issue, so I would seek it out as long as it don't cost you too much for an attorney to consult with you. Good Luck! I would be careful on some of the things said, I would not step on to someone Else's property without a Law Enforcement Officer, that is trespassing or theft if you take the calf if that person calls the authorities, you can see where this would get sticky for you. Do it the rite way if the guy refuses to give you the calf back. Yes it is your calf, you still have to show that. The guy did not steal the calf from you, you guys agreed for him to pay at a later date, same thing if you buy a car and default on a payment, that is not a criminal issue. Take the rite steps and you will be fine, go over and speak with the guy first, if that don't work go talk to the brand inspectors and get legal advice.