Is it permissible to marry a foreign woman for a specific period to obtain citizenship, knowing that this is the only means of livelihood?

If a duration is specified in the contract, after which it will be annulled, then it is a mut'ah marriage (temporary marriage) and is invalid. If the contract does not specify a particular duration, but the husband intends it, then it is impermissible due to the deception involved and its resemblance to a mut'ah marriage. The same applies if the wife knows that the marriage will be annulled after the husband achieves his objective of obtaining citizenship.

Marriage to non-Muslim women contains corruptions, especially if it is for the purpose of obtaining citizenship, because the woman may consider it a commercial transaction, which could threaten his honor and raise issues concerning the lineage of children.

As for the questioner's statement that there is no source of livelihood except that, it is incorrect, for the sources of livelihood are many and guaranteed by Allah the Almighty.