"Provided [assuming] that it is safe to travel in September to Europe,"
If you want to assume that, feel free. If your assumption is correct, and if your assumption that Europe will be open and welcoming to tourists, where's the risk in buying a non refundable ticket?
The problem in your question is this: You have no data (and neither do we) that European travel will be safe by September, or that Europe (Germany and Greece) will be ready to restart the tourist machinery. I have a ticket to and from Denver from my home here in Nevada at the end of September. I have no idea if non-essential air travel will be approved by then. The tickets are refundable, so there is no loss if I decide to cancel, but I am not seeing positive signs that either this country or Europe will be open by then.
By all means, though, feel free to bet on an unknown future if you can afford to lose the money.