Thank you for your question.
Since you are the Plaintiff in an FCRA case, whether the defendant's request for your social security number is XXXXX the scope of discovery is debatable, depending on the specific circumstances of the case.
What I mean by this is that an objection that the request seeks to violate your right to privacy and seeks information outside of the scope of discovery is one that is made in good faith. A motion to compel is the only way for the judge to rule on whether your objections are valid or not. If you are trying to work in good faith with the other side (which it sounds like you are), then you should not worry about sanctions. The threat of sanctions sounds like the defense attorney
is trying to bully you.
Making your answer "subject to" the objection is perfectly fine as well. If he wants to get a ruling on the objection, it is his right to do so. But, it is your right to make the objection, if the objection is made in good faith. Don't withdraw the objections based on his threat. Tell him to get a ruling from the judge and tell him that you are trying to cooperate, but that you do not believe he is entitled to the information requested.
Sanctions would probably be the cost of the motion (between 500 to 1500).
SSN's are not always disclosable. If the information that the defendant seeks may be obtained without the use of your SSN, then the defendant should obtain the information that way (which is less intrusive).
Please let me know if you have further questions.