Company not giving Reliving letter

Hi All, I had worked in a company for 2yrs. I hd given resignation with 15 days notice period but in my offer letter mentioned if i give resignation i hv to serve 3 month notice period or on payment of gross monthly salary and if company terminate me company will give 1 month notice period. Question:1. My company is not giving me my reliving letter for not serving 3 months notice period they are demanding full month gross salary but i told them i will pay you 15 days salary because i hv served 15 days notice period but they are still demanding full month gross salary. what i hv to do, giving them full month salary or 15 days salary. 2. in my offer letter it is mentioned that office timing is 8:30 AM to 5:30 PM but i hv worked more than 9hrs and they never pay overtime. My question is if they are doing according to offer letter in that case can i claim my overtime money because There is nothing mention in offer letter about overtime. 3.After my last increment they deduct 2000 p/m from my salary as retention amount without prior intimation which will be paid after one year if i serve for next one year. Can raise this question. Please help me above points.