I assume you intend to do something with the money the object that is been paid would receive.
Three situations:
1. Both objects are owned by the same owner:
It's possible to set up your own 'pay' system, by using any inter-object communication method (llIM, llEmail, llSay, whatever fits).
Instead of the sending object actually paying the receiving object. It sends an 'acknowledgement' of the payment. Then the receiving object uses that information to do something with the owners money. (Permission debit should be set).
2. Both objects have different owners:
You need two-way communication. Object 1 telling object 2 something has to be done with money. Object 2 giving the keys of the receiving parties (avies) of the money. Object 1 then doing the actual payment.
3. Receiving object is an arbitrary object without any scripting.
Get the ownerkey instead of the object key. And pay that.

Ofcourse, it's wise to do some encryption/hashing on the communication, to make them less vulnerable to 'outside' attacks.