I'm highlighting some points that stood out to me:

1. At the time of resignation (around 1st April 2024), OP was fully aware of the 90-day notice period. Therefore, if they have to serve their full notice period, they will be relieved by 30th June 2024.
2. Knowing the above, OP accepted the offer with their potential new employer, and informed them that they would join on 20th May 2024, effectively serving only half of their notice period. (Not sure whether MNO knows this, or the amounts involved for buyback. If not, they will soon come to know when they ask for confirmation of the buyout amount from ABC). If nothing changes, then as on 20th May 2024, the contract between OP and ABC has not been discharged, and therefore OP is liable to be sued for contract violation.
3. The company reserves the right to buy out the remainder of the notice period. Hence, this cannot be enforced.

Options, in order of preference:

1. What's done is done. Apologize and mend your relationship with ABC at the earliest and convince them for an early release. Promise and complete the necessary business formalities such as knowledge transfer and even interviewing and training your replacement.
2. Apologize and inform MNO that due to ongoing commitments at ABC for a specific client, your joining will be delayed by 45 days (or find out the earliest date that ABC will relieve you and mention that). At this stage of the process, MNO likes you, and would understand, especially considering it's an international hire.
3. Decline MNO (since they would otherwise start their visa paperwork), but feel confident of your abilities to get a job in this cold market, and utilize the MNO offer as a bargaining chip to get a different offer.
4. Abscond, but God-forbid, if you ever want to be hired in India again due to some unforeseen circumstance, and they ask for letters of past experience, then you'll be in soup because you definitely won't get one from ABC if you've left them like this.

Good luck!

I'm highlighting some points that stood out to me:

1. At the time of resignation (around 1st April 2024), OP was fully aware of the 90-day notice period. Therefore, if they have to serve their full notice period, they will be relieved by 30th June 2024.
2. Knowing the above, OP accepted the offer with their potential new employer, and informed them that they would join on 20th May 2024, effectively serving only half of their notice period. (Not sure whether MNO knows this, or the amounts involved for buyback. If not, they will soon come to know when they ask for confirmation of the buyout amount from XYZ). If nothing changes, then as on 20th May 2024, the contract between OP and XYZ has not been discharged, and therefore OP is liable to be sued for contract violation.
3. The company XYZ reserves the right to let OP buy out the remainder of the notice period. Hence, this cannot be enforced.

Options, in order of preference:

1. What's done is done. Apologize and mend your relationship with XYZ at the earliest and convince them for an early release. Promise and complete the necessary business formalities such as knowledge transfer and even interviewing and training your replacement, and ensure a smooth transition for ABC
2. Apologize and inform MNO that due to ongoing commitments at XYZ for a specific client, your joining will be delayed by 45 days (or find out the earliest date that XYZ will relieve you and mention that). At this stage of the process, MNO likes you, and would understand, especially considering it's an international hire.
3. Decline MNO (since they would otherwise start their visa paperwork), but feel confident of your abilities to get a job in this cold market, and utilize the MNO offer as a bargaining chip to get a different offer.
4. Abscond, but God-forbid, if you ever want to be hired in India again due to some unforeseen circumstance, and they ask for letters of past experience, then you'll be in soup because you definitely won't get one from ABC if you've left them like this.

Good luck!