Here is an excerpt from the Foreign Affairs Manual of the US State Department:
9 FAM 402.2-4(B)(10) (U) Lawful Permanent Resident (LPR) Issued Nonimmigrant Visitor Visa for Emergency Temporary Visit to United States
(U) A lawful permanent resident (LPR) may, in some cases, need to get a visa more quickly than obtaining a returning resident visa would permit. For example: a permanent resident employed by a U.S. corporation may be temporarily assigned abroad which requires him or her to remain out of the United States for more than one year. They may be issued a nonimmigrant visa for the purpose of traveling to the United States for urgent business meeting and Form I-551 need not be surrendered. The relinquishment of the I-551 must not be required as a condition precedent to the issuance of either an immigrant or nonimmigrant visa (NIV) unless DHS has requested such action. You may wish to limit and annotate the visa to reflect the nature of the LPR's travel, and to provide additional information to ports of entry.
However, the law doesn't seem to explain what conditions apply to a person admitted on such a B-1/B-2 visa. Indeed, the situation seems a bit contradictory: a B visa can only be issued to a person "having a residence in a foreign country which he has no intention of abandoning", but if that's true, then the person has abandoned their residence in the US, and cannot avail themselves of the benefits of LPR status. But if that's the case, why would the consular post not require the I-551 (green card) to be relinquished?