AWS Billing After Removing a Member Account from an Organization

When you remove a member account from an AWS Organization, the billing responsibility changes immediately at the point of removal.

In your specific case, since you removed the member account on October 20:

• The management account will be charged for that member account's usage from the beginning of the month until October 20.
• The removed member account becomes a standalone AWS account immediately after removal and is responsible for its own bill payments for any usage after October 20.

AWS handles this split automatically in the billing cycle. The management account is only billed for the portion of the month when the account was part of the organization (October 1-20 in your case). The former member account will receive its own bill for the remainder of the month (October 21-31).

This is similar to how AWS handles accounts joining an organization mid-month - the billing responsibility transfers at the exact point of the membership change, not at the end of a billing cycle.
Sources
questions about removal of accounts from organization: | AWS re:Post
Removing a member account from an organization with AWS Organizations - AWS Organizations
Effective billing date, account activity, and volume discounts - AWS Billing