I want to know that Admission in engineering college through donation is legal or not, will you please provide some information regarding to this??????

The Admission in engineering college through donation is legal, every college has 15% seats are reserved for the management quota

But I would suggest you to take admission in the engineering college through the entrance examination, it will cheap and beneficial for you.

Through the entrance examination, you will save good amount of fee. For your reference here I am telling you the names of Engineering Entrance Exams in India:

JEE Advanced Indian Institutes of Technology Joint Entrance Examination
JEE Main Joint Entrance Examination Main
AIEEE All India Engineering Architecture Entrance Examination
BITSAT Birla Institute of Technology and Science Admission Test
ENAT EPSI National Admission Test
VITEEE Vellore Institute of Technology Engineering Entrance Exam
NAT National Aptitude Test
GATE Graduate Aptitude Test in Engineering
PACET Planning and Architecture Common Entrance Test
ATIT Admission Test for ICFAI Science and Technology
ISAT IIST Admission Test
WAT Written Admission Test
MERI Entrance Exam Marine Engineering & Research Institute Entrance Exam
AMIE Associate Membership of Institution of Engineers
NEAT National Engineering Aptitude Test
NATA National Aptitude Test in Architecture
ECET FDH Engineering Common Entrance Test for Diploma Holders ECET