"My heart is so full of every emotion one could think of," donor Amy Pauvlinch wrote on Facebook.
Nico Rivera has a rare disorder called Alport Syndrome and is currently on dialysis.
"I have shared your letter with my staff, and they are working to see what help they can provide,” Trump wrote back to the 8-year-old boy.
"The fact that I’m alive and am going to have at least 20 more years with my kids, that’s the best thing in the world."
She's now recovering from her surgery.
A sick child may die because his father's criminal record prevents him from donating his kidney.