Liability for bounced cheques in case of joint signatory authority

I was a partner in a company with joint signing authority for the bank account. I sold my share and the memorandum of association changed. However, due to some reasons or busy schedule, I could not change my signing authority from the bank.

My partner issued me a letter that for all released cheques, he will be responsible. Now, in the current situation, some cheques are bounced and police called me.

Please advise, what should I do? Will this letter and change in memorandum protect me?

My partner is still accepting his liabilities for all bounced cheques but he has no money to settle, and I am stuck. Kindly advise.