فَالْعَيْنُ مَرْهَى لِرَهْنِهِ وَيَدِي تَقْصُرُ عَنْ أَنْ تَفُكَّ مَرْوَدَهَا
The eye longs for the pledged implement, while my hand cannot redeem it.
The eye’s need is placed against the hand’s inability. “My hand falls short” is a figurative way of saying that the speaker lacks the means to pay. The contrast turns an economic problem into a vivid bodily image.