There is no pending purchase. It was already made last December. The investment for JUUL is already on the books at the purchase price, so a write-down is possible.
The FTC approval is needed only to change MO's stake from non-voting shares, into voting shares. They have the economic interest, and risk of loss, regardless of whether they get that or not.
The voting rights would give them enough ownership control under accounting rules to adopt the equity method of accounting for their JUUL investment. A non-voting investment might not.
Therefore it is largely the opposite of what you assume. Non-voting shares might be required to be written down solely on the basis of market price (i.e. mark to market accounting). If they obtain FTC approval and have a voting equity interest, they would only write-down their stake once they conduct impairment tests, which would include longer term cash flow and profit projections for JUUL.