sab aatish-e sozandah-e dil se hai jigar aab
be-.sarfah kare .sarf nah kyuu;N diidah-e tar aab
1) from the burning fire of the heart, the liver is entirely water
2) generously/unprofitably, why would the wet eyes not make an expenditure of water?
Notes:
be-.sarfah : 'Unprofitable'. (Platts p.203)
.sarf karnaa : 'To expend, spend (anything, in or on, ...; to disburse; to pass; to use or employ (in or on)'. (Platts p.744)
be-.sarfah : having opened the heart, with generosity
In this verse he has mingled opposites with such excellence that at first glance one doesn't realize it. The heart-burning fire has made the liver into water. (For 'the liver to turn to water' means to be in great trouble or grief; keep this in mind too.) That is, the thing that is burning the heart-- that same thing is turning the liver to water. The liver is the source of blood; now, when the liver itself has turned to water, from where would blood come into the eye? It's clear that nothing but water would flow out of it.
But for the water flowing, there are two more causes as well. Tears are being shed, so that the fire that has been started in the heart would be extinguished. That is, the liver's turning to water, which is a result of a fire starting in the heart-- that same thing will also do the work of extinguishing the fire in the heart. The second cause is that the burning, pain, and grief that exist in the heart because of fire being lit there-- due to them, tears are flowing from the eyes.
Another point is that when the liver was settled and established, in the eyes there was an abundance of tears of blood. Now, when the liver has turned to blood, then there will be an abundance of inappropriate watery tears. The opposition of be-.sarfah .sarf karne me;N should be kept in mind. If we assume the meaning of aab to be 'glitter, gleam' (from weeping, the eyes' aab steadily goes away), then in the two lines an 'iham of sound' [iihaam-e .saut] and an 'iham of affinity' [iihaam-e tanaasub] are both created, and aab (meaning 'glitter, gleam') and aatish (with regard to glitteringness and brightness) there's a 'visual wordplay' [ri((aayat-e na:zar].
