Upon motion of the Director of the Bureau of Prisons under 18 U.S.C. § 3582(c)(1)(A), the court may reduce a term of imprisonment (and may impose a term of supervised release with or without conditions that does not exceed the unserved portion of
the original term of imprisonment) if, after considering the factors set forth in 18 U.S.C. § 3553(a), to the extent that they are applicable, the court determines that—
(1) (A) extraordinary and compelling reasons warrant the reduction; . . .
(2) the defendant is not a danger to the safety of any other person or to the community, as provided in 18 U.S.C. § 3142(g); and
(3) the reduction is consistent with this policy statement.
In 2007 the US Sentencing Commission clarified what it viewed as “extraordinary and compelling reasons”:
(A) Extraordinary and Compelling Reasons.—Provided the defendant meets the requirements of subdivision (2), extraordinary and compelling reasons exist under any of the following circumstances:
(i) The defendant is suffering from a terminal illness.
(ii) The defendant is suffering from a permanent physical or medical condition, or is experiencing deteriorating physical or mental health because of the aging process, that
substantially diminishes the ability of the defendant to provide self-care within the environment of a correctional facility and for which conventional treatment promises no substantial improvement.
(iii) The death or incapacitation of the defendant’s only family member capable of caring for the defendant’s minor child or minor children.
(iv) As determined by the Director of the Bureau of Prisons, there exists in the defendant’s case an extraordinary and compelling reason other than, or in combination with, the reasons described in subdivisions (i), (ii), and (iii).
The system for managing Compassionate release is different, but it's there and the USA isn't as barbaric as you might expect from some of the more rabid comments you may have heard.
So, the US would have sent Megrahi home.