After. The qualifying relationship must exist at filing, and a petition submitted before closing describes a subsidiary that does not yet exist. Prepare everything in advance so the petition can go in the week closing occurs.
Closing first, filing second, travel third
Hypothetical example: the acquisition is scheduled to close on the first of the month, the seller's manager leaves at the end of that month, and the owner wants the South Frontenac manager in Pennsylvania in between. The petition cannot be filed until the Canadian company actually owns the plant, so the realistic sequence is to assemble the year-abroad evidence, organizational charts, duty descriptions and U.S. entity documents beforehand, file within days of closing, and consider premium processing if the gap must be short.
The manager may visit the plant as a business visitor to observe and meet staff during that window, but he cannot run it or perform productive work until he is admitted in L-1A status. Build a contingency into the seller's transition agreement, for instance retaining the departing manager for an extra sixty days, rather than assuming the immigration step will be faster than it is. Note also that the year abroad is measured backward from the filing date, so a delay in closing does not help a transferee whose qualifying year is already old.