Each state's law is different, so it will vary depending on your state and whether there is a particular statute that governs these types of agreements. However, assuming no specific state law, under the common law, you have an oral agreement to sell the bull for a certain price. The devil is in the details and the general rule is that you sell the entire bull unless you specifically and clearly reserve an interest. Without seeing each email correspondence it would be difficult to tell whether you clearly stated your intent to reserve an interest when the bargain was reached. Because it is an oral agreement and there appears to be an ambiguity about the details of the agreement, a court (under the common law) would allow in the evidence (emails leading up to the bargain) to try and determine what were the details of the basic agreement. At that point it is a legal determination on the facts appearing in the emails, what each side says is the bargain, etc. So, the final result is that it depends . . . . on a lot of things, state law, evidence, etc.
Long story short, is it worth the PITA factor to try to reserve a lifetime interest and collect later on? For example new buyers may come up with excuses like bull is "leased to a friend in another county or state," will have broken unit, various health issues, alleged to be dead, etc., so it seems that it would really have to be worth it to reserve the semen interest for the life of the bull. Maybe so many units would be better and collect it right away. The comment above on breed association requirements for registration was a good one too that I had not thought of. JMHO, but interesting case.
