My guess is that your initial retainer agreement talks about reasons either of you could withdraw from the case, and that this might be one of those reasons.
Maybe try to get another attorney to take over? Make sure the second attorney knows about the first, so they can decide whether to get a full waiver of any fees the first attorney might try to recover from a settlement.
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In some cases, the contemplated legal services are not the same in theory as what's required in practice. Some client matters get very complicated through no fault of the client, and that changes the manner in which the attorney can handle the case going forward. As an example, if your case was contemplated as being a relatively simple personal injury matter, but suddenly exploded into a multi-party action with cross-complaints, significant motion practice, etc., then it may be the case that your attorney has suddenly found himself expending far more resources on your case than he anticipated (or you anticipated, for that matter.)
In that case, your attorney can advise you that to continue the case, you need to supply some money for the costs of the case (which is my guess as to what's happening here.) Your original retainer should have some language to that effect, or at least contain a provision whereby the terms can be renegotiated should the case turn out to be more complicated or costly than originally anticipated.
If not, you're always free to refuse to pay going forward. That may require you to find new counsel, however.
The foregoing is not legal advice, and nothing in the foregoing shall be deemed to create an attorney client relationship. If you feel you need to speak with an attorney regarding your issue, it is recommended that you contact an attorney with expertise in your area of inquiry. The information related above is purely for informational purposes, and should not be acted upon without speaking with qualified counsel familiar with you specific situation and the laws related thereto.