Trade relations are strengthened by disallowing migration. Migration takes cheap labor from one nation and puts it in the other. This means that the cheap labor that would have benefited the poorer country will instead benefit the migratory worker and the wealthier nation instead. This benefits both nations, but the wealthier nation moreso, as the migrant workers face a higher cost of living and thus put the money they make back into the local economy rather than their poorer home country. The benefit is in remittences, or monies sent back to families, however as previously stated, they spend more money to live in their host country. Migratory restrictions strengthen trade and trade relations.
Migratory restrictions do not damage trade between countries because a country will not simply boycott another country for having strict restrictions about migration. Such restrictions could also improve trade by making it more difficult for citizens of one country to leave for another, encouraging the seeking of self employment and of trading with neighboring countries to procure goods.