The force of “li-yakūna lahum ʿaduwwan wa ḥazanan” cannot be determined from the letter lām in isolation. The verb fa-ltaqaṭahu, the agency of Pharaoh's household, and the subsequent enmity and sorrow show that producing an enemy was not their purpose.
This is the contextual clue. Treating the particle as though it carried one mechanically fixed sense everywhere would misread the account.
Both verbal and particle metaphors require analysis of the prior meaning or relation through which the expression comes to be used figuratively.