"Spoiling" digital cash

[georgemw at speakeasy.net]

An idea just popped into my head, I was wondering if
anyone had thought of this before. Most likely someone has,
and has either proven the idea is impossible or has figured
out how to do it.

Th idea is,  when buying some good or service with
digital cash,  the customer first forwards the cash to the vendor
in some transformed way such that the vendor 
can't yet spend it, but can verify that it is
good cash of the correct amount, and that 
the customer will no longer be able to spend it.

The idea is, if the vendor follows through on his side,
the customer will supply the additional information the
vendor will need to redeem the cash.  The customer can still
rip the vendor off by refusing to do so, but he has no incentive,
the money's already gone for him.  Conversely,  an
unscrupulous "vendor" could in principle trick a customer
into throwing away money on nothing,  but he would gain no profit
in doing so.

I realize the same effect is trivial to achieve if you have a mutually
trusted third party willing to act as escrow agent, but if you
don't, is there a a way to build this into a transfer protocol?

Thanks,
	George