Assalamo alaikum wr wb.
I am revert Allhamdulilah from 7 years but just from year and half i live back in my nonmuslim country in sha Allah. I was working with my boss for few months and find he ever late to my payment and dont fix that . I start with understanding to get payed on 30 date of the month...for 3 months i end up at 5 date next...wanted to keep money from me what i disagree becouse i need them and becouse we work without contract ,since he do not insure me...why should i?...Anyway i spoke with good intantion to him all his clients working with me ware happy to do so , they even personnaly called him to say good words for me , even i ware my hijab and thats first scary them,as christians...I find out with the time many bad practise from my boss which i dislike and decide to leave ,but i did it with 3 of his clients...They really wanted to work only with me...once my boss removed me from one of them and she did everything to get me back...so now i work with them on my own...i do take same money as i taked from my boss,i do not take his % now , nor want to go this way i just wanted to take my money on time and can insure my living and count properly my expansis...I do believe that someones sin do not excuse me for doing sin too,Did you think i have to repent for taking his clients? We still work with my clients together , they are happy pay me less , i am happy they pay me on time . Did i have to do Istagfar?
Salam

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, is His slave and Messenger.

Your question included several matters, the answer to which can be summarized in the following points:

First: It is impermissible for an employer to delay the payment of the salaries beyond their due time. The Prophet said: “Give a laborer his wages before his perspiration dries up.” [Ibn Maajah, Al-Albaani: authentic due to the collective versions narrated through different chains of narration]

Second: what this employer did, withholding the employee’s salary or some of his rights and denying them because there is no written and documented contract between them, is considered a grave misdeed. Abu Hurayrah narrated that the Prophet said: “Allah, The Exalted, Said: ‘I will be an opponent to three types of people on the Day of Resurrection: … (one of which is:) a man who employs a laborer and gets the full work done by him but does not pay him his wages.” [Al-Bukhari]

Third: there is nothing wrong with dealing with some of your former employer’s clients, and there is no sin on you because of it. It is not incumbent on you to refrain from dealing with them and offer them the lawful services they requested that are not related to your previous job. Moreover, there is no sin on you for benefiting from the experience that you gained from your previous job. What is impermissible is for an employee to disclose work secrets and the like.

Accordingly, you bear no sin – Allah Willing – for dealing with those clients or others who request lawful services from you, even if they are your former employer’s clients.

Allah Knows best.