We had pimped out our Kunekune boar a couple of times, until he came back having had a bit of a doing after one such jaunt
We couldn't prove it, but to see a pig who usually had to be rubbed and scratched for at least 10 minutes before food time, suddenly recoil from an outstretched hand and actually run away from my OH was too much for me

Now he very seldom goes out (only to very trusted people) and instead we take visiting sows and gilts here and that works better for us

Sorting out the practicalities with a written agreement would definately be the way to go - something that states price and the responsibilities of the hirer (feed, vet care if needed etc etc etc) but think carefully BB - I've heard stories of stud boars moving from farm to farm til it ended up that the owner didn't actually know where he was

If you have a seperate area that you could have approved as an 'isolation unit' it might be worth going down the visiting sows route instead ?
When we have a visiting sow we charge for service (as Lynne says, the price of a piglet) and the price of feeding and on the occaisions we've hired boars from others we pay a service fee for each pig he serves and are responsible for providing his feed while he's here - we've never had occaision to call a vet for any visiting pigs, but I think it would depend on the nature of what was wrong as to who's responsible for it, I guess it would be the hirer if the boar was in good health when he left your holding - but suspect you'd need a vet report stating that to be able to claim any recompense if something did happen

HTH a bit ?
Karen