@Alex419
Adding to @Linda108's good explanation -
Don't ever reimburse a guest outside the system. If it had been a longer booking the guest would still receive part of the costs depending on how many unused days. The system would calculate this and reimburse the guest automaticaly. Meaning you'd be out of money twice.
If the guest wasn't entitled to money and you wanted to pay them something anyway, you can always choose to reimburse them for an amount when they cancel. If they decide not to cancel but leave, and you want to reimburse, then you can do that through Change reservation by overwriting the amount. The system then reimburses the difference betw. paid amount and changed amount.