Dear [Name],
I am writing to apologize for the monetary mishaps that have occurred in our business relationship. I understand that this has caused you a great deal of frustration and I take full responsibility for the situation.
I want to assure you that I am doing everything in my power to rectify the situation and make sure that it does not happen again. I understand that trust is the foundation of any successful business relationship and I regret that I have broken that trust.
I am sorry for not being more diligent in my financial responsibilities and for not taking the time to double-check the accuracy of my calculations. I understand that this has caused you to question the reliability of our business partnership and I want to assure you that I am taking steps to make sure that this does not happen again.
I am also sorry for not being more communicative with you about the situation. I understand that this has caused you to feel as though I am not taking your concerns seriously and I want to assure you that I am doing everything in my power to rectify the situation.
I understand that this has caused you a great deal of frustration and I want to offer my sincerest apologies. I am doing everything in my power to make sure that this does not happen again and I am committed to rebuilding the trust between us.
Please accept my sincerest apologies for the monetary mishaps that have occurred. I understand that this has caused you a great deal of frustration and I am doing everything in my power to rectify the situation.
Sincerely,
[Your Name]