Hard to say without knowing the details of the issue more specifically. One would hope that the seller is going to fulfill his promises and that any sale of the property is subject to the agreement he has with you. The problem is you don't have a firm agreement yet with the seller, it's not signed much less recorded and it sounds like the issue is more about a misrepresentation with the seller then a problem with the boundary needing to be fixed because someone plotted it wrong. If the adjacent property were sold and your problem is really for misrepresentation by your seller then you might be left with only being able to sue the seller for money damages.
Your lawyer should have someone checking his messages and covering for him. Try e-mailing him or leaving a message. Normally property doesn't sell overnight especially if there is a title company checking on things and they see a pending suit for the boundary line issue, but you never know.
If you can't get a hold of your lawyer you might want to contact another lawyer while you wait for your lawyer to return.
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Since you have a lawyer, he or she is the person with whom you should be talking, and as was suggested, there is likely someone covering during your lawyer's absence. As a general principle (which may or may not be entirely applicable to your situation, depending on the detailed facts), if your dispute involves where the boundary really is, the new purchaser of the adjacent ground would still be subject to your claims. But if it is a question of where the boundary should be, and you are trying to get the seller to adjust the plat or otherwise relocate the boundary to give you more land than your original deed gives you, once the seller has sold the adjacent property, the new owner would not be bound to honor the terms of the (proposed) settlement, if not completed.
This comment is general in nature and is not intended as legal advice. It does not create an attorney client relationship and obviously is not confidential. You should contact an attorney in your area who can review with you all of the relevant facts and give you specific legal advice.