It works just as you would expect it to except you tax summary will show you are now a phone retailer. Your sales have increase by 100.60 rather than your purchases decreasing by 100.60.
If you enter the same physical transaction into Manager as a debit note your purchases will decrease by 100.60 and your sales will remain then same. Alternatively you can add in some more payments or receipts with negative values or Journal entries using tax codes and Manger will report arbitrarily large sales and purchases.
Manager not being able to reliably calculate you actual total sales or purchases is a bit of an issue if your tax department ask you to report either, and has penalties for submitting incorrect information.
Doesn’t really make any difference if you happen to live in a country where your tax department only asks you to report the difference in sales and purchases, or doesn’t really care what numbers you submit.
Managers accounting errors become even more obvious if you look at other payment or receipt transactions