Based upon RMM Player's Guide, page 48.
The idea is that the thief has to acquire the treasure by adventure. It need not ever even hit his physical possession, if he's instrumental in his group acquiring it. I'd not give him XP for stuff he wasn't involved in, but if he cajoles a share out of the others, yes, he gets XP for it.
EG: Joxer opens the treasure trove's locked door, and Aeolius goes in and raids 100 gold worth of loot whilst Joxer stands watch. I'd give Joxer the 100 XP for that, even tho' Joxer never touched the loot. Why? Because Aeolius wouldn't have been able to do so. Later, Joxer tackles a guard, almost as if by clumsiness, and Aeolius grabs the 1GP value Vase, and a 2 GP sword, and they make their escape. I'd give 2 XP for that. But, when Joxer's asleep, and Aeolius snags the chakram of doom, no bonus for Joxer. (Note: Aeolius is a lyrist...)
Remember: Old School Gaming requires application of large doses of common sense in place of detailed lists of rule and exceptions.