Like others have said, talk over the details with the other party and write up a contract. Business is business, if the other party does not want a contract then do NOT do the loan. IMO the real purpose of the contract is to set down on paper the parameters of the loan so when the time comes to end the loan and split the babies, there is no forgetting of the details.
I have completed one loan and am in the middle of two others.
things you might want to include...
Who provides which specific animal
Where the loan will occur (which party will have both animals together)
Time frame (including quarantine time?)
EXACTLY how babies are to be split including specific selection process. For example one loan I am in is a 50/50 split. Babies will be divided into 4 groups based on visual phenotype and sex. Selection process will be who goes first for each group and alternating until all babies in that group are gone. Yes, that does mean the person who starts picking in a specific group could get one more baby than the other person. Can't divide babies in half. I can't stress enough that this needs to be crystal clear so there are no hard feelings. Of the four groups I go first on two and the other party goes first on the other two. That way we each have the same chance of getting an extra baby or two. Let the boa gods decide how the litter comes out.
What to do with any ODD or defective babies! What if a new morph pops out?
What to do in the case of illness - will be different if the loan is around the corner vs accross the country.
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Thanks,
Dave Colling

www.rainbows-r-us-reptiles.com
0.1 Wife (WC and still very fiesty)
0.2 kids (CBB, a big part of our selective breeding program)
LOL, to many snakes to list, last count:
26.49 BRB
20.21 BCI
And those are only the breeders 
lots.lots.lots feeder mice and rats 

