«لَمْ أُخْسِرْهُ»
I did not shortchange him.
Lam negates a jussive imperfect with past-time meaning. “Ukhsir” is the jussive verb; “-hu” is its object pronoun.
«يَوْمُهُ أَخْسَرُ مِنْ أَمْسِهِ»
His today is worse than his yesterday.
Akhsar is a comparative adjective; min amsihi gives the comparison. Observing both the word form and its syntactic position makes the poet's logic clearer.