You should have a lawyer experienced with easements review the easement. If it is an easement in gross it is personal to the grantee and may not be assigned. If the easement is appurtenant to the grantee's property (which you should assume for now) action on your part will be required. You should get advice before the foreclosure sale and certainly before the end of any applicable redemption period.
This response is for informational purposes only and is not legal advice, nor does it rise to an attorney client relationship. You should not act upon this information without seeking professional legal counsel.
If I understand your post correctly, the foreclosure sale has already occurred. I agree with Mr. Guerin that prompt action on your part is called for. You should gather all of the information available, including a copy of the sheriff's deed with the legal description of the property that was sold and schedule an appointment with an attorney familiar with real estate matters. Your plan of obtaining a deed from the departing homeowner that releases his claims to the easement is a good one and should be pursued.
You haven't identified the other parties involved in this question so I cannot determine whether I may have a conflict in this matter. Should it turn out that I have an attorney-client relationship with any of the other parties, my response to this question will not prevent me from continuing to represent an existing client.