Cheaper to buy a female pastel I think LOL
Since he's not a good friend and somebody unlikely to back out or keep all the eggs for himself and give you some story how she never took etc...I wouldn't bother.
However, a contract, to be legally binding, should be witnessed by somebody reliable so you have a third party to vouch that such a contract existed.
I would demand a signed paper (on letterhead of the practice) of a qualified herp/snake vet saying the female is in perfect health (and get one for your male(s) as well). Agree on whether eggs will be incubated by him, you or split between the both of you, and agree on how to handle 'odd' eggs (ie if 5 eggs are laid and are all good, who gets the 'third' egg?).
Or, if you let him incubate all the eggs, and there's an odd number of pastels (I am assuming you have a normal snake to breed with his pastel), who gets the odd one? Then there is who gets first pick of the babies.
i would also put in the owner of the female will need to get a vet inspection to confirm she's gravid or not...just in case he wants to claim she never laid any eggs so he can sell them himself.
There should also be some agreement on what to do if the unexpected happened...like one of the snakes dies...(if both were vet certified as healthy, this shouldn't happen but never know). If a vet inspecting the sick/dead snake finds the cause is not something it 'caught' from the other snake (ie thermostat fails and temperature in the cage spikes to 120F and cooks the snake...)Or if he gives you the female for x period of time and she dies while in your care, or vise versa if you bring your male to her instead of the other way around. Will the 'host' be expected to pay the cost of a replacement snake of same age/size/morph? Or just a set cost?
Given the fact its getting to the point now that most ball pythons who have been cooled or prepped to breed have already done so, it may be better to find a yearling female pastel and buy it, then maybe get her up to weight for next season or the season after that, much less hassle, especially if anything should go wrong.
I would see this being worth the hassle to get a contract done up and signed by the both of you and witnessed (who signs it as well) and have three copies with signatures (originals, not photocopies, that is, make three copies, then sign all three together). If you were breeding a higher end morph, like a super pastel spider het pied to a pied pastel or something.
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PHLdyPayne