Opposite of a deposit refund

Solution 1:

I did put this in as a comment, but the more I think about it the more it seems to me it's the best option, so here it is as an answer that can be voted on.

I think forfeit is commonly used in respect of security deposits which are not refunded. To my ear this can function as a past participle/adjective even without the -ed suffix, but I suspect many might prefer the more "grammatical" form.

EDIT: As suggested above, will be forfeit has been gaining traction against the "regular" past participle / adjectival form will be forfeited. As this NGram shows, the noninflected form now accounts for 2 out of 5 BrE usages. The shift is less marked in AmE, but it's happening there too.

Solution 2:

Is this like a security deposit (when renting property)? Since a security deposit isn't usually binary—you can have part of it returned and part not to pay for damages—would you be better off thinking of this as "amount returned" (and zero means you kept it all)?

If it really is binary in your case and you don't like kept, consider retained or not returned.