tasya kaalena sat-putrau
vavRdhaate bhavaaya tau
dharm' aarthaav iva bhuutaye
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Those two true sons, in time,
Grew up to do him proud,
Just as, when his investment is great,
Dharma and wealth pay a noble person well.
Again, the easy way to understand this verse is that, because of bringing up his two sons, one with an inherent inclination towards religious, spiritual or philosophical truth (dharma) and the other with an inherent inclination towards the sensual, material or substantial (artha), the king is like a noble person who is rewarded for effort or money invested in two contrasted areas: either the area of religious, spiritual or philosophical truth, or the area of material substance / financial wealth.
But the real meaning of the verse, as I read it, is that the king, as a wise father of two sons -- each of whom was good or true, each of whom had the buddha-nature -- attended in the round to the psycho-physical well-being of each son, and he was rewarded accordingly by seeing his sons prosper.
A signpost to Ashvaghosha's real intention in this verse, then, was provided back in 2.55, in the suggestion that the means to become calm and content lies in a balance of the faculties of thinking and feeling.
Understood like this, the meaning of dharma might be the young Gautama's idea, just before his enlightenment, to give up ascetic end-gaining for enlightenment (3.5); and artha might be the choicest, most wholesome rice that he then ate. Without the true thought, the Buddha's enlightenment could not have happened. At the same time, it couldn't have happened without the wholesome rice.
Again, true dharma might be the traditional method of sewing the seven-stripe robe, and artha might be thread and cloth, not to mention needle and good sharp scissors -- in which case it is never a question of selecting either dharma or artha.
Sometimes it is a question of selection, sometimes it is not. In Canto 4, the Buddha puts Nanda in a position where he is forced to choose. But here the king stands resolutely in the middle.
Those two good sons in time grew up for his wellbeing, just as Religion and Wealth increase for the prosperity of a noble man great in his undertakings.
In time his two good sons grew up to do him credit, just as dharma and wealth bring profit to a gentleman with ambitious projects.
tasya (gen. sg.): of him
kaalena: ind. instr. in the course of time
sat-putrau = nom. dual. sat-putra: m. a good or virtuous son ; a son who performs all the prescribed rites in honour of his ancestors
vavRdhaate = 3rd pers. dual perfect vRdh: to grow, grow up
bhavaaya = dat. sg. bhavaa: m. becoming, being ; worldly existence , the world (= saMsaara); (with Buddhists) continuity of becoming (a link in the twelvefold chain of causation) ; well-being , prosperity , welfare , excellence (= shreyas)
tau (nom. dual. m.): those
aaryasya (gen. sg.): m. a respectable or honourable or faithful man ; a man highly esteemed , a respectable , honourable man ; a master , an owner
aaramba-mahataH (gen. sg. m.): great in his undertakings
aarambha: m. undertaking , beginning ; a thing begun ; (in dram.) the commencement of the action which awakens an interest in the progress of the principal plot ; haste , speed ; effort , exertion
mahat: mfn. great, abundant, extensive
dharm'aarthau (nom. dual): dharma and wealth
dharma: dharma, duty, law etc.
bhuutaye = dat. sg. bhuuti: f. existence , being ; wellbeing , thriving , prosperity , might , power , wealth , fortune ; Welfare personified (= lakShmii)