Can I generate one off receipts with Donation Manager?

Donation Manager is designed to be used for annual receipts. There is a use case for one off receipts, but unless you had a low volume of donations it would make it reasonably cumbersome.

The process would essentially be that for each donation you received, you could do a receipt generation in Donation Manager. Then at the end of the year when you went to do your annual receipts, you would need to roll back all your receipts for that year, and include them all again in the annual receipt run.

If you are using Xero, you would be better off to use their Remittance Advice functionality and modify the layout, to suit a one off receipt.