Deb, did this client of yours have you do a vendor package and did you sign the agreement with them, like what most larger companies do these days?  If so, is there something in there about no finds? If not, I would charge them.  Or perhaps give them so many no finds per month, then after the limit, charge half of the retreival fee.  I know that sucks, but trying to get something may be better than the nothing you have right now.  If you did sign an agreement and there is something in it about the subject, perhaps one can be revised to include something.
Just an idea.