@Gabriel4142 Had this been me, I would have explained to the guest that I would accept a change for 1 month versus 2, stating that the reduced rate that they received for their month plus booking was far less than the price that would have been quoted for the three weeks that they were able to stay. I would show them the price that they would have paid for the three weeks in comparison to the month long booking, and stress that at a minimum, I should receive the month income, and that I will forego the income for the second month.
Hopefully the guest would comprehend that they were being "forgiven" for blocking the calendar for the second month and that it would be difficult, if not impossible, to rebook those dates.
Unfortunately, as you did accept their change request, you are going to have to return the $300 for the nights that they did not stay, as well as "lose" the income from the second month unless you are able to rebook those nights.
As @Mike-And-Jane0 pointed out, the change request should have alerted you to the reduced or refundable income when the guest submitted it.