«وَدَرُّكَ يَفِيضُ وَرَدُّكَ يَغِيضُ»
Your bounty overflows; rejection diminishes the hope of the petitioner.
Darr can evoke the flowing of milk and, figuratively, abundance. Radd means return or refusal. The pair fayḍ / ghayḍ contrasts overflow with depletion.
Both the sound and the imagery make the petition urgent. Generosity produces a flowing surplus, but a rebuff dries up expectation. The writer turns a complicated emotional argument into a short, symmetrical sentence.