I personally find it refreshing to read an example that is not "A deposits $10, B withdraws $10".
In Postgresql, the default ISOLATION LEVEL is "READ COMMITTED", which by standard, allows "NON-REPEATABLE READ" phenomena.
When Transaction on the LEFT is COMMITTED (adding 100 to balance), the Transaction on the RIGHT will add 400 to a balance of 1100, and not to a balance of 1000 (reading NON-REPEATED value).
You can do better than leaving this kind of comment that does not contribute in anything.
Have good weekend.