Hi @AirTraveler - there isn’t a built in way to do this in Airtable, i.e. take a snapshot of a field value and save it somewhere else. Some options though (ignoring the manual option of copy and paste)
Write an Airtable script that copies the current value and writes it to the previous balance with a button click (you’ll still need to remember to run the script though)
Automate on a periodic basis using Zapier
Automate using a Google Script and the Airtable API
Hope this helps!
(If this has answered your question, please mark as the “solution” - if not, post again! Thanks!)