>>> Is ARIN willing to take back the block and refund the fee paid?
In a bizarre twist, this could theoretically cost you money. =-)
Kim and David Conrad (et al) have made it clear that the registry
is charging for the work that goes into making the allocation, not
for the address space itself. When you "de-allocate" space, they have
to go through more work (although probably much less) to edit their
files, databases, whois server, etc.
(all that said, I'd bet they won't charge for this service =-)
In any case, the question is not relevant as ARIN (just as the NIC)
can only allocate space, but can not guarantee routability.